I clinched the final playoff spot in my league with 3 games to go. I drafted plenty of minutes and got blessed with a good rookie, so all of my players are at 100% and most have recommended minutes greater than their min/82.

My question: Should I continue resting/under-playing my best players now that I have clinched? Will their restedness allow me to play them heavier minutes in the playoffs? Thanks!!

not really - it's more or less a reset in terms of you cant stockpile as you suggest - there is however a benefit to getting the first rd bye in that you can bank some portion of those first rd minutes and overplay dudes in later rounds