Hi VickiVM,
As part of our research for the Unofficial Guide to Walt Disney World, we've measured the wait times at every attraction in every park for the past two Christmas years. Based on those data, here's what I'd suggest:
First, note that the parks (especially the MK) may open an hour earlier (e.g., 8 a.m.) every day, so EMH may actually begin as early as 7 a.m. The official park hours do not reflect EMH, but you can check them to get an idea of what to expect. I'd check those hours just before you leave, as they're apt to change between now and then.
The Magic Kingdom will be mobbed every day, so I'd take advantage of EMH there.
It's probably a good idea to use EMH at the Studios, too.
Use EMH at Epcot at your discretion - the capacities of the attractions are such that Epcot handles large crowds very well. If you wanted to skip EMH at Epcot, your touring and waits probably would not vary significantly.
I'd avoid EMH at the AK during Christmas. You may end up waiting longer for most attractions than on a non-EMH day.
If this is your first time at the parks, or you plan to do a comprehensive tour of the parks, a good touring plan is essential.
Hope this helps!
Len