We ALWAYS hit the EMH park 1st, making sure we get there early, and hit the main attractions that the kids want to hit first (Test Track @ Epcot, or Space Mountain at MK), and then once the crowds hit (about 2-3 hours after early opening), head to one of the other parks.
We find that the EVENING EMH are probably the ones that have the biggest crowds. We found that at EPCOT, for instance at 9:00pm (we went in Feb.), the fastpasses were gone. Again, Evening EMH can be crowd intense, as fewer of the attractions are open, and more people are attending the park.
At least that was my experience in Feb of 2005. It will be interesing to see if anything has changed when we go in Feb of 2006.