If you ask me, I think that keeping it closer to 20-30% is more acceptible, especially during a boss fight, for the simple fact that the tanks are getting spam healed, and if the healer were overly concerned about overhealing, they might cancel their heal and the tank will pretty much die.
I play a priest to which I have raid healed with, and if you're talking for a full-raid overhealing %, I think 20-30% is an acceptible range. Anything greater than 40%, imo, then the healer needs to adjust how they're playing. They're effectively wasting half their mana by overhealing the raid.
Take that with a grain of salt though, because sometimes you just *have* to spam heal, and you will always get a lot of overhealing from it.
Please consider my point of view, as I'm primarily a mage, but I do have a priest to which I have experience on, just not as much as everyone else.
EDIT: (I agree with what CheshireCat says) Pretty much says everything I did, with a better explanation to it. =P