Although Venice has a lot of history to offer, I recommend that you absorb this history in the off season. However, it seems that Venice floods in the off season, so I can't really think of a good time to go. When I went, it was like a day at Disneyland - too many people and too little space. I think there were actually more tourists than there were Venetians. In fact, there were too many tourists to really take in the romance and the atmosphere of the city. It might not have helped that, at the time I went, there was an ongoing strike amongst the city's trash collectors, so there were piles of trash sitting outside doorways and behind restaurants. There was even substantial gondola traffic, which I didn't know was possible. Overall, Venice was kind of a disappointment for me.
Thanks for the info. I can't stand when I have a certain picture in my mind of a relaxing visit to a place just to find 1000s of people had the same idea! Too bad you can't know in advance how many tourists will be there.