I also wanted to add that if your going for a holiday week it really pays to do some research on which park is best for which day. There are several sites out there that offer this kind of info. The vast majority of people visiting WDW don't research such info. Most people tend to go with which ever park is offering EMH in hopes of getting a few more hours of park time in.
For example, last year during Christmas week we ran into a friend of DH's at a resort. They had been to MK that day (or the day before), and he was telling us that it was WALL to WALL people. He said he couldn't even take pictures of his kids because he couldn't back up enough to get a good shot. They bailed on the park because it was impossible to get anything done.
We went to MK the very next day and it was such a quiet day. Crowds were not bad at all. DH's buddy had been to MK on an evening EMH day, and so did lots of other people, hoping to get in a few extra hours of park time. We went to MK on a day no EMH was offered, and a lot of the crowds went to parks that were offering some sort of EMH.
Not only does time of year matter. Which park you choose on which day of your trip can make a huge difference in the crowds you encounter.