For morning EMH -- the parks are not that crowded. In fact, even without the EMH, the parks are not that crowded the first couple of hours after opening. If you can make it out there for the opening, it's a good thing to do since you can accomplish quite a bit in those first 2-3 hours, before the crowd and heat descend on you!
For evening EMH -- Our experience in May was that the parks were crowded. No fastpasses are available, and only a limited number of rides were operating, making for longer lines and grumpier guests. Instead, we hit the parks with EMH in the afternoons, and the crowds were actually quite light, since most people may be resting up in preparation to hit the park that night. Also, when we went to a park the morning AFTER its evening EMH, the crowds were very light.
Have a great trip!
Linda