We have been there last week of May or first week of June four of the past five years. We also have been there for my DH's birthday on July 21 the past two years. This is an EASY recommendation. It is slightly less hot, a good bit less crowded and rains much less in May than in July.
We postponed our regular early summer trip until July in 2002 b/c my oldest child was graduating from high school. We came home saying we would never go in July again unless it was the only time we could go. It rained pretty much every day and was more crowded than we were accustomed to. We zipped down there for a weekend the next year, though, because our APs were expiring and we wanted to ride Mission Space before they did. It was hot, crowded and rained and stormed so badly our arrival day that we couldn't get to e-night as planned. Once again, we came home saying, "never again in July."
GO IN MAY! Avoid MK on the first Saturday in June -- otherwise that week is great!