Too many variables to say. If you're on H Thok, yeah, healers have to be spot on but generally speaking, healers need only heal as much as the damage that's being taken which, at least partially, is on everyone doing the right thing.
I play shadow priest and the "worse" I play, the better DPS I do. Ever done a flex in your heroic geared guild who go in there with their mains? You'll most likely see the sloppiest play ever. We'll stand in crap since it's trivial, we'll take risks, we'll do crazy stuff since we know we won't get 1 shot and if we don't get 1 shot, our healers, being overgeared themselves for the content will keep you up.
So many examples of what I can do to boost DPS but put that on healers. Keep in mind moving for Shadow Priests is bad (our DPS drops by a lot while moving so we try to limit it).
Protectors: Say I decide to be slow on moving out of the group when I'm being mind seared because I have trinket going or even want to just finish my cast so I lose the least amount possible? Good for DPS, bad for healing.
Heroic Sha of Pride: Say I don't run off to get a far away fissure in the ground and wait for one to be closer so I don't lose DPS (meaning we have more of the little pools appearing under us doing damage)? Say I AOE all the adds that pop instead of letting melee/tanks just naturally cleave them? AOE on adds will increase my dps but since melee can cleave them naturally, I do more to benefit the fight if I stick on the boss.
Galakras: Meh, I'm doing huge damage so why not stand in that fire arrow a little, 2-3 ticks the healers can handle it...but my DPS is higher!
Heroic Iron Juggernaut: Meh, moving sucks and those saw blades are hard to dodge sometimes...I know I can take the hit and live, I'll just not move.
The examples are endless not to mention use of defensive cooldowns (and yes, I do mass dispel on Thok, why not help right?!). Healers need only heal what damage is being taken...no more, no less. I've seen some bad healers that can't handle it, I have but generally speaking, in a stable raid group with relatively equally skilled people, the wipes almost all come from personal mistakes of not handling a mechanic correctly or not avoiding damage.