Hi Cary
Last summer while we were there in July, in addition to negotiating the usual summer crowds, most Austrian schools get out around the 10th (there is a site that Jo from Germany directed me to for researching when exactly the specific areas of interest got out, but I can't remember the link).
Anyway, we found Vienna very easy to navigate while Salzburg was significantly more crowded. We didn't see any appreciable increase in volume after the school vacation date so I wouldn't worry in that regard. Mostly because most sights are in a smaller geographical area versus Vienna.
Even with the crowds, as Steve accurately points out, you're not talking space mountain kinds of crowds. We didn't encounter any lines for sights, even for places like Mozart's residences in Salzburg, museums in Vienna, etc... - really (except maybe for bathrooms).
But there definitely were alot were alot of people roaming the Getreidegasse. If you're trying to get isolated pictures without crowds at locations like the Do-Re-Mi steps at the Mirabell garden - go early. Biggest crowd was at the Augustiner Brewery. Still worth going there anyway.