Time to post my emh guidelines.
None of these are hard and fast. All are situational
1. If you don't want to read a bunch of rules, avoid emh parks entirely and stop reading right here.
2. If you are staying off site, avoid emh parks entirely. You can't take advantage of emh.
3. The emh decision is not independent of the decision to get hoppers. And if you're not getting hoppers, consider avoiding emh parks. If you get into a bad situation, you can't hop out of it.
4. During slack times of the year, you should consider emh. The hours are short, parks aren't crowded (10 extra percent of a low number is still a low number) and the extra time helps. You should consider this even if you don't have hoppers.
5. During the most crowded times of the year you should consider emh. All parks are packed. What's a few more and the extra hours help. Note that in the busiest time periods, closures apply.
6. If Disney drops a surprise emh on you within 2 weeks of your departure or worse, while you're down there, strongly consider avoiding that park that day if you can. Disney expects crowds. That's why they decided to put it in late. If you're stuck with those adrs that you just can't pass on, don't panic. Make yourself a good plan and you can still have some fun.
Morning emh
8. There is less time for the emh, but it starts from a zero line condition.
9. If you have hoppers and otherwise intend to go take a rest at the hotel and hop to another park, by all means consider morning emh. You lose nothing in the transfer (you were going to do it anyway) and you have extra time. I see no downside here.
10. If you intend to attend the emh and directly hop to another park, you're going to lose a lot of time in the transfer and your new park will not be at a zero line condition.
11. Don't go to a park that has a morning emh without attending the emh.
Evening emh.
11. In general, the later the emh, the less crowded it should be. The drawback is you are less likely to want to get up the next morning.
12. These do not start at a zero line condition. But they last longer.
13. Sometimes, you can hit a park with an evening emh early in the morning when it opens and hop out if need be. Also, early in the morning the day after a late evening emh was just held can be good too.