The morning EMH work well for me as far as helping me to experience the greatest number of attractions with the shortest possible wait times.
The evening hours I like for the fact that they allow me to extend the amount of time I can spend in the parks, especially when the parks close early. However, I don't find them very useful for rides. I find them more useful for providing extra time for meals, shopping, shows, parades, and fireworks.
On short trips I like to hop, on longer ones we usually pick a park and stay there all day.
I've never had an issue staying all day at a morning EMH park. I think this is because in general,
1. Fewer people take advantage of the morning hour because they don't want to get up early. People tend to stay up late on vacation, not get up early.
2. Half of the ones who do come hop away by lunch.
3. The planners staying offsite seem to follow the advice given to them of avoiding the EMH parks.
This all seems to sort of even out the whole crowd thing to me. Yes, it is more crowded than the non EMH, but not ridiculously so. By doing all the headliners in the morning, the crowds don't really affect me in the afternoon because by then we have moved onto meals, shows, and other low wait / no wait attractions anyway. I'd say if the crowds are going to be high to begin with, hop away because every little bit helps, but if not, I wouldn't feel like I had to hop if I didn't want to.
However, I'd never stay all day at an EMH evening park. No advantage. I'd only hop there at the end of the day, and then I wouldn’t expect to get a lot done or to walk on rides unless the park stays open after midnight and you are willing to stay until 2 or 3 am.