A lot of this is park dependent. At the MK, if you blow through your FPs early, you will most certainly be able to pick up more FPs, but they will be for rides such as PoTC, HM, Laugh Factory, maybe Buzz. On a crowded day, those might be very valuable. If you wait too long at the MK, you might lose out on the ability to get some very valuable passes. But at Epcot, blowing through your FPs early on a crowded day will not necessarily help you. If you book for 9:00, 10:00 and 11:00 and use your third at 11:01, the odds of getting that extra Tier 1 FP for either TT or Soarin' is pretty slim. And even if you get one, it will likely be for a late time, thus ending your quest for FPs. It is probably better at Epcot to do RD for the Tier 1 ride that you cannot FP, and then hit up other rides from 9:20-11:00 while the crowds are concentrated at the "Big 2". Then, use your FPs between 11:00 and 3:00, or thereabouts. So there is no one "right" answer.