We have been to WDW in June three times and the crowds build as the morning progresses and do not diminish much, if at all later in the day.
The best strategy is to get to any park at opening. You can get so much done in a few hours (EMH or regular opening) before the really big crowds arrive later in the morning.
Last June we watched Mickey's Philharmagic in the Magic Kingdom with about six other people right after park opening.
Also, last June the evening EMH was way too crowded to enjoy. The park with the evening EMH has really big crowds arriving from late afternoon on. It's a good idea to skip that park later in the day.