Train Reservations - are they needed? How far in advance?

First, you are required to have a seat reservations on ALL trains in Italy except the Regionale trains. So it depends on the train whether or not a reservation is required. (Obviously) Regionale trains are the cheapest and slowest - think bus on rail - that stop at nearly every town. You just get on. No seat available. Stand until someone gets off at the next stop.
Second, if the real question is -- Do I need an ADVANCE reservation so that I know I have seat on the train? That answer is mostly likely - NO. Trains rarely if ever sell out in Italy on a routine bases. Special weekend, holiday, etc., probability of a sell out is higher but still unlikely.
Third, the only reason, but it is good one, for purchasing a ticket in advance, on-line, is to take advantage of any discount fares that may be available. But, if you haven't purchases advanced tickets by now, I doubt if any discount fares would be available today. So just buy them when you get there, maybe the day before you want to leave. NEVER buy a Regionale ticket online.

It depends on what train it is whether you will have seat reservations or not. Regionale no, others yes. You dont have to buy of them in advance if you dont want to.. The only benefit is that there might be cheaper tickets available on the fast "freccia" trains. Those tickets often sell out and possibly arent available now anyway. The only fast train you are likely to use is Florence to Venice, others are regionale.

Be sure you validate the tickets for regionale trains or you could be subject to hefty fines on the spot!

Thank you so much. Yes...I learned the hard way that the 'fast' trains DO sometimes sell out. I had checked train schedules weeks ago & thought 'no problem' getting from Rome to Florence mid morning...when I checked a few days ago, they were SOLD OUT during the times I wanted to travel from the Termini station....had to book an Italo train out of Tiburtina.

The schedule I am concerned about (Florence to CT) lists a 'fast' train #9764 from Pisa to LaSpezia....should I try to reserve that now, or wait until I get to Florence??

Also, the CT to Venice leg includes a 'fast' train #9418 from Florence to Venice...again, should I reserve tickets now, or wait??

Well, if you know EXACTLY when you plan to travel, get the tickets now (not regionale, though, never buy those in advance unless its part of a trip using fast trains). Then you can just breeze into the RR station 10-15 minutes before the train leaves. Otherwise you have to stand in line to buy, and hope the machine likes your credit card.

I am curious. What date/time did you wanted to go from Rome to Florence? If it is critical for a specific train - date and time - then reserve it now.