Are advance train tickets needed?

Really comes down to flexibility versus peace of mind. Given your trip is during Italy's peak summer period, to and from highly traveled cities, I would certainly recommend securing tickets ahead of time. You can check route times via Trenitalia site, then reserve specific seats and print the tickets from the comfort of your desk. Often times they'll offer discounted rates (booking ahead of time), however since your trip is so soon those economy rates may well be gone. Even if paying full fare, I find value in the certainty of knowing we're confirmed for a certain route at a preferred time. Makes for planning the rest of that day's itinerary so much easier than rolling the dice and 'hoping' there's availability.

Only if you want to save money.

Some people feel stressed about not knowing which train they'll get on and whether they can all sit together. There is no deadline to buy tickets, but finding 4 or 5 seats together will be more likely if you book ahead. Other travelers feel stressed about having to be on time for a specific reservation. Any tickets that you buy for these routes, which are all served by medium-speed and faster trains (except the Napoli-Sorrento commuter line or the Rome airport train), will be for reserved dates, times, and seat assignments.

Too late to save any real money. With your group size you could have made sizeable savings by booking 120 days out on all legs except Napoli to Sorrento.

As there are five if you I would still book now to make sure you can all be seated together on the services you wish to catch.

krista,

You don't "need" to buy the tickets in advance, but as the previous reply mentioned significant savings may have been possible with advance purchase. At this late date it's likely that all the discounted tickets are now sold out, but you can easily buy tickets at local stations either at a Kiosk or staffed ticket office, as the price will be the same. Most of the trains you'll be using have compulsory seat reservations so one advantage of buying in advance (even if the price will be the same), is that you may have a better chance of getting seats together. If you wait until the last minute, you'll have to take whatever tickets are left and some members of your group could be sitting in different cars.

One important point to mention is that the seat reservations for the "fast" trains you'll be using are specific to train, date and departure time, and can only be used on the ONE train listed on the ticket. If you board another train by mistake, you'll face hefty fines which will be collected on the spot!

A few comments on each of your rail trips....

• Venice to Florence - use either the Freccia or Italo fast trains. Note that your departure station will be Venezia Santa Lucia and your arrival station Firenze SM Novella. Many cities have more than one station, so it's important to know the correct ones when buying tickets.
• Florence to Rome - the simplest solution would be the Freccia trains from Firenze SMN to Roma Termini (Italo uses different stations).
• Rome to Sorrento - you'll first need to travel to Napoli Centrale via Freccia trains, and then transfer to the local Circumvesuviana commuter train for the last segment to Sorrento. You'll need to go downstairs from Napoli Centrale to the Garibaldi station for that. The Circumvesuviana can be a "shock" for some when they first see it, as it's somewhat colourful and has a bit of a reputation as a haven for pickpockets and scammers. Be sure to wear Money Belts anywhere in Italy and don't access them where others can see. If you'd rather not deal with the Circumvesuviana, you could consider hiring a private driver such as the Monetti Taxi listed in the guidebook.

One final point to mention is that if you use any Regionale trains, Buses, etc., the tickets MUST be validated prior to boarding on the day of travel, or again hefty fines.