Hi jog58...
I've always felt the crowds weren't that bad in May. I mean, if you go expecting no lines and no crowds you'll be disappointed. It's busy, but I've always found it very tolerable and if you go with a plan, you'll have no issues. However, I'd be inclined to avoid IOA or USF or even Sea World to a degree like the plague!!! It seems that every year in May every school group in central Florida ventures to these locations at various times. If you have your heart set on going to one of these places, do it, youll still enjoy yourself but be prepared just in case.
Hope this helps...