A number of websites suggest that you can make a reservation for parking at the Tronchetto garage, but after contacting the garage (email) we were told no reservations were needed. Do we understand things correctly? We don't want to waste any time searching for parking!! Is it necessary or possible to make reservations?
I have not parked at the Tronchetto garage but I do not think you have to have reservations to park there. For a cheaper alternative you may want to consider parking in Mestre across the street from the train station. Much cheaper per day than Tronchetto. The cost for us two years ago was 4,50 euros per day and the ride on the train into Venice is only a couple of euros and takes just minutes to get there.
You don't necessarily need to book, but you can if you want to. It all depends on the time of the year that you are going. For example, will you be arriving during a weekend in August? If so, I'd get a reservation through the VeniceParking.it website.
For more details on parking in and around Venice, take a look at this great article on the EuropeforVisitors.com website.
We parked there with no reservations in October. It took forever to get in and out and wouldn't have been any quicker with reservations.
I would park on the mainland and take a train in. It's expensive and time consuming to park in Venice.
The Tronchetto is huge, so unless it's a day Venice will be packed, you're unlikely to need a reservation. The website suggestion is a good one to check. E-mail is iffy because it could have been answered by a functionary who just didn't want to be bothered or understand your question.
Parked here for several nights with no reservation.
Also be aware that there are many break-ins in these parking garages. Theives know people are in Venice all day at least. Stow away all your valuables.