For trains with optional seat reservations it’s very dependent on the time and date you take the train. So please mention travel dates and time for exact advice.

Quite often you consult the expected seat availability of the train on the websites/apps of the train operators. So you can decide if you go for a seat reservation a bit later, like one or two weeks before you travel.

If you take the train on a random blue tuesday, in the middle of the day, far away from any holiday or longer weekend, I wouldn’t bother to get a seat reservation.

If you plan to take the train on a Friday or Sunday, you can expect larger crowds, because more people will be travelling to go on a weekend or a small city trip. During high season or a holiday period (Christmas, Easter, Ascension weekend, July-August) I would definitely go for seat reservations, since a lot of people will be travelling. If you want to sit together as a group of 4, reserve during high season.

You can check your train on the Cesky Drahy website for a forecast. The little icon to the right of the train number shows how busy it’s expected to be, but take it with a pinch of salt.

Don’t expect to get 4 seats together around a table without reserving. Much easier to split up if it’s busy.

https://www.cd.cz/en/default.htm

Example for Budapest-Prague tomorrow, looking not too busy. As ​@BrendanDB says, on a Friday in Summer it will be a different story

We are planning around august time, and based on your guys experiences would August weekend be bad option to leave it unbooked?. Then how about the weekdays of August

Also, lets just say I dont book seats as a group of 3 or 4. Can we just go anywhere in the second class  train cabins?. What happens if a person comes and just says its there seat and the cabins are full?

On an August weekend I would definitely reserve seats, especially as a group of 4.

Seats that are reserved should have an indication - either paper slip or digital display - showing the reservation. Pick seats that have no indication, or are reserved later in the journey. You can also ask the train manager.

If somebody comes along and says you’re in their seat, you can ask them to show you! (This happened to me yesterday in UK, on a very busy train. Somebody told me I was in their seats, and showed me the reservation. It turns out they had reserved seats on a different train!)

So on a august weekday lets just say from budapest to vienna. I save the trainn journey in the app and just go to the train stop on the day of th trip?. I know this is a pretty simple question but should I do anything extra if im not looking to book a seat, that means just saving the journey and going to the train as a group of 3 

Regarding a transfer in between cities like this one for example, am I good if I dont book for 3 people?

Yes, that’s pretty much all you need to do, i.e. show up at the station, save the journey into “my trip”, add the journey to your pass, and you’re ready to go (see here for a step-by-step: https://www.interrail.eu/en/interrail-passes/interrail-mobile-pass/getting-started ). If all passes are on the same phone, you have to do this separately for all group members.

If you have no reservation and the train is full, you may need to stand, or move to different seats or cars. I have taken this train (Budapest-Prague) a number of times for parts of the journey (Breclav-Prague), and yes, it can be very full, so you may not be able to sit together as a group (or sit at all).

The situation is not very different if you make a transfer somewhere. Except here it might be even more important to stay in contact with the other group members so as to assure everyone makes the transfer :-)

Yes, the connections are no issue.

However as said, you risk getting split or worse standing as you will be travelling in peak peak season (15th August is even a holiday).

Is a 3€ saving per person/train worth for you? That's the question. The prices shown on Rail Planner are not correct if you book through the railway companies.

By the way, the train in the screenshot (EC 113 Prague-Cracow) is a through train. There's no change in Bohumín. Reservations can be booked for around €3 on cd.cz, max. 60 days in advance and are mandatory for the 20 minutes between Bohumín and Zebrzydowice. In summer though, it can get busy so just book the whole journey.

Hmm but in the rail planner it’s asking me to pay twice for the journey. Should I just buy a separate ticket and not use Eurail?

Booking reservations via Eurail is more expensive in most cases. Also their timetable planner is not very reliable.

Just book where it's cheaper, e.g. on cd.cz (Czech railways). Eurail also add a €2 booking fee per person and train, so they're almost always more expensive.

This page gives an overview of the best ways to book pass reservations:

https://www.seat61.com/interrail-and-eurail-reservations.htm

I took a look at it and now I get it, I wasn’t aware you were able to just buy reservations only in the website (Haven’t had that much experience with european trains). Crazy that eurail offer booking reservation twice and in the czech website its another thing. Thanks for the info. I have couple questions if you dont mind sorry for pestering 

1. just to be safe all I need to do is have the eurail pass and book the seat ‘reservation’ not ticket and I should be good with seating right?. 

2. Eurail adding a 2 euro booking fee, I’m planning to take 12 train rides on different travel days so will eurail charge me 2 euros for all them or charge them individually
3. Lastly in the pics listed below, rail planner is telling me to change trains from Berlin-Amsterdam but when I checked deutsche bahn there isny any transfer. So should I just purchase a seat reservation on deutsche bahn and follow their schedule with regards to this train

If you have a seat reservation where required, you should indeed be good to go, independently of where you get the reservation from.

Not all trains require reservations, and many can also be reserved through other ways without incurring in the fees. However, for those where it's not the case, Eurail offers the service to book the reservations through their portal which has indeed that €2 fee per reservation.

When looking this far ahead, timetables are not yet complete. It is likely there will be a direct train to Amsterdam Central station unless engineering works prevent it from running.

Regarding second question, So its 2 euros on each ride per person then…..

Yes, on top of the reservation fees. It's generally cheaper to book with the company itself when available (either online or at their ticket offices).

This is one of the reasons you should not use the Rail planner app to plan. Just use it to manage your pass. The timetables you'll get directly from the operators are much more likely to be correct. By the time you travel, the Rail planner app will normally have been updated and you'll probably find a timetable that matches what the operators say.

So as of right now I should just look back on rides I planned  on the app and verify them with the operator?. Also what do you mean to manage my pass like see how many rides fit into my travel days?

That's a good idea.

During your trip, add a journey to your pass before you board a train and generate the QR code.