Train advance booking?

It is rare except on special occasions for any train to be filled so I would not worry about being able to get to your next site on the day you would want to travel. Obviously mid-day trains will have more travelers than earlier or late. The ONLY reason for advance purchase to to take advantage of any discount available. You should not have any problem if you bought your tickets the day before. There are 23 trains between 8 am and 8 pm each day between Rome and Venice. I am sure that you could find a couple of empty seats on one of those train the day you want to travel. I would not worry about it.

But understand that on high speed trains in Italy, you can't just get on and look for an empty seat. You have to have a seat reservation. Any ticket you purchase, in advance, online, or in the station, will come with a seat reservation. If you are using a rail pass, you still have to go to the counter or an automat and purchase a seat reservation. If there are no seats available, you won't be able to get a reservation or get on the train.

If you know you have to be in a certain place on a certain day, you might want to just book the tickets. I assume you mean August 2015; you've already lost some flexibility in which tickets are available. I don't know if money is no object for you, but waiting until departure time to purchase changes a 2nd cl 27€ ticket to an 80€ one (Venezia > Roma), and a 9€ one to a 26€ one (Roma > Napoli). I don't know how many are in your party, but those €€€ can add up.

Since these are major train runs, they may use extra cars...or not. I have had to stand on trains in Italy during August. Not fun. And that was nearly 20 years ago. There are waaay more 'regular folk' traveling now. Personally, I'd reserve. But will you lose 2 days of your vacation waiting for the next train with available seats? No.

Although August will be busy, you shouldn't have any trouble buying tickets the day prior to travel, either from a Kiosk or staffed ticket office. However as the others mentioned that will mean paying Base Fare which is the highest price tier. For travel this August, it's likely all the discounted tickets are already sold out so there's no advantage to buying advance tickets. If you find that a particular train is fully booked, that's not a problem as there will be another along in an hour or so.

One other point to consider is that for the routes you mentioned, you can use either Trenitalia or Italo (although Italo often uses Tiburtina station in Rome and not Termini). Having two choices provides more options.

Note that your tickets (seat reservations) will be specific to one train, date and departure time, so can ONLY be used on that ONE train. If you board the wrong train by mistake, you'll likely face hefty fines.

I am sorry but 20 year year old experiences doesn't apply anymore. The only train you would have to stand on is Regionale train because this is no limit to the number of tickets sold and no seats are reserved. But you are not looking at any Regionale trains so the probability of standing has nothing to do with your concerns about getting on the train. If they sell you a tickets, there is a seat for you on the train. Again, there is pretty of capacity so don't worry about it.

Thank you everyone for your responses! It's too bad I missed out on the super discounted advance tickets Eileen mentioned, but it looks like I can still get some savings by booking now. For peace of mind and saving a few Euro, that's what we'll do. Thanks again.

Just be sure you completely understand the restrictions that come with the discount tickets.