Well, that's a tricky one.

On one hand, giving the characters a crunchy reason to give up encourages acting with reason. (As opposed to counting hp.) On the other hand, it's not very cool to have big, nasty foes die in couple of rounds when the players've figured out the system. (Or vica versa.)

How about this the following?
"At a dramatically appropriate moment (as determined by the DM), a character may count a flat-footed or pinned creature helpless for the purposes of performing a coup de grace."

Encourages roleplay, makes pinning a bit more desirable than I see it as, and is abuse-proof.