My own theory is that party days are good days to go. We have been to parties 4 times where we spend the entire day in the park and stay through until the party.
I do think enough people avoid the park on early closing days to make it worthwhile. People who want to see Wishes or the night parade check the schedule, and soon find out they can't see it on those days. Otherwise how do SO MANY people end up in the park on the days those things ARE offered? During that time of year, the parties can close the MK as many as 4 nights a week. The 3 nights you can see Wishes or the night parade are PACKED. So I like to avoid those nights.
We did
MVMCP twice in 08 after being in the park all day both days. Both had very reasonable to low crowds. But then it was December, so it might have been that way anyway!
We did
MNSSHP twice in 09. That Tuesday we got there close to park opening and it was CRAZY BUSY packed to the gills. I never did figure it out. Since they were setting record temps that week, it make the park much less than "magical" and we opted to leave and come back later for the party.
Two days later on Thursday, we were there for AM EMH opening and stayed all the way through. It is the quietest day I have ever spent at the MK (well.... up until the party, and even that wasn't horrible). I had always heard about walk-on rides but had never seen it. MTMRR and Splash had 5-10 miute waits at 1 PM. I had never seen anything like it.
When we stay straight through, we take a slower pace, and plan 1, maybe even 2 sit down table service meals. Then we start the party by picking our favorite "must do" and starting the line at 6:00 or so. We sit down, rest, chat, and end up first in line for something that generally requires a long wait.
There are lots of restful things you can do in the park. See the parade, ride TTA a few times through, find a bench. If we go back to the hotel, my daughter is too old to nap anyway, and I just toss and turn TRYING to nap, so we might as well just stay in the park.
It works for us. And it will be much easier this year since we'll be there in December. If it's hot when you're there, it's WAY harder to stay all day.