This might vary from place to place, but I see this text on the Doge's Palace website:
"It is possible to book online only reduced price tickets for children aged from 6 to 14, students aged from 15 to 25 and over 65. Other reduced and free tickets are instead issued, upon presentation of the requested documents, directly at the Ticket Office."
The good news is that the website does mention that there's a Rolling Venice discount. The bad news is that you can only get that discount by standing in the ticket line. I know the popular Secret Itineraries Tour at the Palace sells out in advance. I don't know whether the same is true of basic entry tickets.
You need to check all the pertinent websites carefully so you know the situation. That will only be important at places in such demand that sellouts or significant ticket lines are possible. That would include San Marco. I've seen lines at the Guggenheim a couple of times, but they weren't terribly long, and they may just have been people holding tickets who were waiting for their entry time. The Guggenheim is very popular, however, so there could potentially be an issue there. I've been to the Accademia twice, both times in September, and there was no hint of a line there.
Sometimes a special exhibition at a museum is so popular that tickets need to be nailed down in advance, but I've been to a lot of museums in Venice without encountering a significant ticket line, aside from whatever was going on with the lines I saw at the Guggenheim.
The Rolling Venice discount on the 72-hour transit pass is very worthwhile unless you plan to walk nearly all the time. An individual vaporetto ticket costs 9.50 euros, but the 72-hour pass is only 27 euros (as opposed to 45 euros without the Rolling Venice card).
