Salzburg to Venice - Advance train tickets necessary in off season?

I don't think you'll have much worry about seats being sold out at this time of year, but seat reservations are pretty cheap insurance. Even if the train is SRO, it is unlikely that all seats will be reserved. And, if they are, you'll know because they won't sell you a reservation. You could save considerable with an advance reservation. For instance, the train/bus connection via Villach is 59€ at the counter and as low as 29€ with advance purchase.

Lee, thanks for the help. Do you think a day or two would be enough in advance?

I think the advance purcase discounted fares are only available online, and you must book at least three days in advance, but the best prices sell out early. You should be able to start booking 92 days in advance. Those times are for tickets on German Rail; I'm not sure what it is for Austrian Rail. BTW, the Austrian Rail connection is a train from Salzburg to Villach and a modern express bus from Villach to Venice. I'm sure you could find a seat reservation for the train two days in advance, but the bus is smaller. I'm not so sure. But there is not SRO on the bus. If they let you on, there will be a seat. And, there are a number of buses a day, so there should be something available. There is also an all-rail connection from Salzburg to Venice via Rosenheim, Germany. It takes a little longer, but that connection can be booked on the German Rail (Bahn) website. Put in from: Salzburg Hbf to: Venice Santa Lucia and use Rosenheim as a stopover. There is a connection from Salzburg at 10:09 with a 44 min connection in Rosenheim around noon to the direct EuroCity train which gets to Venice at 6:10 PM. I see fares as low as 49€/person. You could book that connection online, with a reservation for the EC to Venice, now. The train from Salzburg to Rosenheim is a regional train (RE). There are no seat reservations for that train.