Train Tickets in Italy/ quick itinerary feedback.

You don't need to buy any tickets in advance. Trains do not sell out, especially in low season. Occasionally the cheapest class (standard class) may sell out on some morning trains on popular routes (like Rome-Florence-Venice and viceversa) during the high tourist season. In those circumstances, you can either buy a higher class ticket, like premium or business (which never sell out) or simply wait for the next train half hour later.

The advantage of purchasing in advance, however, is that you can secure that way tickets at a discounted fare (Super Economy or Economy). They start selling 120 days in advance of the day of travel, and those deals are often gone quickly.

Those Super Economy and Economy fares, however, come with strings attached: they are NON REFUNDABLE and NON CHANGEABLE. Therefore if you miss the specific train you book, you lose your money. Buy those tickets only if and when you are reasonably certain of your travel plans.

FYI, you might be one of the last travelers on the direct Munich-Venice City Night Line train. It's scheduled to run through December 9. Although winter schedules are not yet published, and an Austrian connection could be an option, German rail is expected to discontinue their sleeper train network at that time.