Best case scenarios...
For morning EMH, go ahead and take advantage of it to ride a few "headliner" attractions before the general public opening time. Then, you can ride the minor stuff while everyone else is fighting over the E-tickets. Then, if you have a park hopper, hit another park that will likely be less crowded since a good many resort guests will still be at the Morning Hours park.
For evening EMH, stay as late as you can. The later it gets, the less crowded it is. I actually suggest getting dinner right around park closing, giving the first rush of people to EMH attractions right after fireworks time to clear out. After that, most stuff will be a ten minute wait or less. The closer you get to midnight (providing the parks close around 9PM), the more stuff becomes essentially walk-on. Around 11:45 one night, we rode test track five times straight without even getting out of the car!
An alternate option for evening magic hours is to skip that park on EMH night, then hit it right at opening on the next morning, since presumably everyone else went there "yesterday" and also stayed up late.
I've heard some people say that the best time to hit the parks is early early early on January 1st, since everyone stayed up till well past midnight the night before, and is either tired or hungover. :lol: