I had the same question last year. We were trying to decide whether to take DS out of school a few days early (last part of May) vs. wait 'till school was officially out (closer to the first of June). Most people who responded said to go with dates in May.
We ended up taking him out of school a few days early and went the toward the last week in May. It was a great time to be at WDW: temps. were milder, crowds were very low. The only down side was Memorial Day w/end, which was really crowded.
NO REGRETS!!! We're looking at doing the same again this year!