We went the first week of May in 2010 and I found the crowds to be completely reasonable to me. (It was 95 degrees, which I did NOT find completely reasonable, but that's why I go in April or October now. )
We have been the week before Memorial day and the week of and I find either very manageable! Not all schools are out yet! We leave May 19th and I can't wait!!! Excited for the 24 hour party even though it will be a mad house that day and night! That is part of the fun!