If you don't or can't select a seat the system will do its best to sit you together. It works 99% of the time. You'll get assigned seat numbers right before or right after paying, depending on the website/train.

If you don't have a reservation at all you'll get thrown off the train* and/or get a big fine.

*complicated between Rome and Florence as there's no intermediate stop...

There are 3-4 hourly trains between Rome and Florence so you've got plenty of choices. Those trains never sell out.

Thanks. I definitely want to reserve a seat, I was just wondering if it was worth paying extra for seat selection. Often the Extras are just extra revenue for the seller, arguably with no benefit to the buyer. Being my first trip to Europe, I won’t take the risk and just pay the extra.

I need to travel Rome to Florence this coming Saturday but don’t really know what time I can get to the station yet. With my 1st class Eurail pass, how close to the departure time do I need to reserve my seat?

There are many trains on that route, so you should always find something.

Unless you very strongly want a particular seat, there’s no need to pay for seat selection. The system will allocate you seats together where possible.

I do it on an airline, to avoid getting stuck in a middle seat, but there are no middle seats here!

As for when to reserve - no need to do it far in advance in Italy, unless you’re travelling on a holiday weekend or something. You can reserve up to 5 minutes before departure, I believe. So just do it on your phone on the way to the station.

There’s always seats. You can check by looking up availability for your route as if you were leaving right now, and you’ll see plenty of availability.

Not sure I understand the process. The Eurail app says seat reservation required but you said "there’s no need to pay for seat selection". Is the 13 Euro for a reservation or seat selection? It sounds like the 13bEuro covers the reservation and I can choose to pay extra to select a seat. 

No 13 Euro is the Reservation cost. To choose a specific seat you pay 2 Euro on top.

Got it. Thanks.