For the parks that have the morning EMH, the early hours will usually be fine. If you are a resort guest and get to the park at the EMH opening, you will generally be able to get a lot done in the first few hours. After that though the parks will tend to be more crowded than the same park on a non EMH morning day.
Some of our best days on our last trip were the EMH mornings at the MK. Got there around 7 and the place was empty. We have pictures and there is no one else in sight.
At night, the first few hours tend to be crowded. Since the park itself may already be crowded, people will just stay longer. As the night goes on, the park will start to empty out. This is especially true when the EMH park is the MK and the EMH starts at midnight. By 2, if you can handle it, the park will be nicely empty. However, if the night EMH starts a lot earlier in the evening, it may not empty out as much.