Hi YankeeFan,
You should plan if:
If you've got a list of attractions you want to see
and at least one of the following two things is true
1) You're going during a peak time (summer, holidays, spring break)
2) You're not going to get up early every morning to be at the park when it opens.
If you're going to do a fairly comprehensive tour of, say, the Magic Kingdom, having a plan will save you up to five hours of standing in line.
Similarly, sleeping in is perfectly normal on a vacation. But if you still want to see a lot of attractions, every hour you spend sleeping in costs about two hours of waiting in line. That is, if you arrive at 11 a.m. instead of 9 a.m., you'll wait in line four more hours to see the same attractions.
If seeing a lot of attractions isn't important to you on this trip, then you don't need a plan. I would, however, try to wander around Epcot in the evenings. It's wonderful.
Len