No, I wouldn't buy the Rail Protection Plan on most train tickets. I would not pay $50 to insure against a possible loss of $400. If you don't actually go on your trip, some tickets are partly refundable if returned within a certain period before the scheduled travel date. If you do go, just try not to miss reserved trains, particularly the first and most expensive one, departing Munich.
While I expected Rail Europe to offer the German advance-discount fares, I see that's not currently the case on the Munich-Venice route, so you would get a better price through DeutcheBahn.
If you don't feel ready to commit now to specific dates and times for each leg of the trip, you don't have to. It's easy enough to buy train tickets on short notice, both in stations and on the German and Italian web sites.