Usually early October is a great choice of time to go, but when you're at WDW over a holiday week-end, it's always more crowded than normal.
I avoid holidays myself, because I made the unfortunate mistake of visiting over Thanksgiving week-end back in '97. But we visit in early-to-mid October frequently and it's a great time to go. I think you'll find crowds significantly more than early May, but maybe not quite as bad as mid-summer.