Is it really necessary to buy and reserve these ahead of time? For example, to go from Milan to Venice or Florence to Rome. Or can I usually just show up the morning of my desired day and get tickets?
Sure, there is always a possibility that a particular train might be sold out but extremely remote. I have never encountered a sold out train in Italy. The exception is night trains.
I have traveled in Italy by train many times and never have had a problem buying a ticket. However, make sure you are not traveling during a holiday season. I would buy my tickets as I go and it is easy. Almost all stations have automatic kiosks that take credit cards and are accessible in about 6 languages.
Buon viaggio,
RB
Richard,
You shouldn't have any trouble buying tickets on the day of departure. You might find it helpful to check the Trenitalia website so that you have an idea on the departure times of the various trains.
I often buy my tickets for the outgoing journey when I arrive in each city (I'm at the station anyway). If you'll be travelling on one of the "fast" trains (ES/AV), the reservation fee will be included in the cost of the ticket. The ticket will specify your Car no. and Seat no.
You might find it helpful to have a look at www.roninrome.com or www.seat61.com/Italy.htm, as both websites contain lots of information on train travel in Italy. Don't forget to validate!
Happy travels!