Now that we need to go to Disney based on DS school vacations, I have found that the end of August is a good time to go. We've gone sometime during the last 2 weeks of August the past 3 years and the crowds are very manageable. We ride standby most of the time without long waits. We still like to get to the parks at opening as we find the first couple of hours to be the best plus as the day goes on it does get hot. Yes, it's hot in August, but compared to the other times we can go based on school vacations (Late June - early Aug, President's Week , Easter week, Christmas week) I think this is the least crowded. We used to love going Labor Day week as the crowds were very low, but when DS began kindergarten that wasn't a good time so we opted for end of August.
We live in NYC so my son doesn't go back to school until after Labor Dar - I know other states begin earlier. In addition to school starting, I think the heat and hurricane / storm potential keeps some of the crowds away in late August.