When to make reservations

I basicly only booked seat reservations when IT was mandatory. If your are travelling alone, finding an unreserved seat is usually not a problem. 

There are some trains with mandatory reservations which should be booked early. (Eurostar, TGV (they get more expensive if the train IS getting full), night trains)

​@elliedooler 

It is impossibke to answer your questions. It depends on the places and regions you visit; some suffer from overtourism, while others are deserted in the summer.

My rule of thumb from my experience

Travel alone => only book the mandatory ones. There were very few occasions when I was lucky to have an optional seat reservation when travelling alone. All of those were on international trains (e.g. Hamburg - Copenhagen, Budapest - Prague etc.)

Travel in a group => Book also the optional ones for longer direct journeys.

You find usually seats when you do your mandatory reservations a few days/weeks in advance. The trains you should book asap are Eurostar trains, Night trains and TGV trains (especially if you want one of those which avoid Paris).

Pretty much my experience. Also you are more flexible without a reservation. (more relevant If you travel alone). 

Most of my overfilled train experiences were on regional trains so far (no reservations possible).

Indeed, except in Switzerland, where a reservation is really only recommended if you're a large group.