This is interesting, because we DID book a FP for Peter Pan on ONE of our MK days, and we went straight to the mountains at rope drop. Rode BTMRR twice with no wait, then Splash. It was a ghost town over there!
The below will show you why people use their FP on Peter Pan - the lines build to "unbearable" FAST.
http://www.easywdw.com/reports13/mk32715.jpg
With the exception of SDMT and A&E, all lines are "short first thing at RD". It's just a matter of what your priorities are.
In MK, there are any number of ways to go. It's much more clear cut in, say, Epcot. We USED to always start off at Fantasyland, but my daughter is 13 now, and doesn't want to ride many of those attractions anymore (Pooh and the Carousel come to mind) - at least they're not a priority anymore.