It is a matter of choice whether you reserve well in advance or simply book at the station. There are many trains, both direct and with connections between Rome and Venice so you will get a seat but they may not be together or mean a different train. Reservations for Italian trains can be made online at OEBB.at (Choose Interrail/Eurail as a discount pass for each passenger. They can also be made at a couple of other sites and if you want those I can post them.

Italian fast trains are branded Freccia and cost 10 euro per passenger, per train. They operate Intercity trains that,whilst express trains they are slower but only cos 3 euro per seat.