Well, the healing won't damage them, unlike in most RPGs, with my reasoning being that you are, in fact, mending the corpse, making it a better corpse for whatever happens to be controlling it right now instead of the original owner. Think of it this way, say there was a zombie with a broken leg, you heal that leg, well, now it's a slightly better zombie 'cause you just healed it you dolt. Though, with your reasoning, I guess there could be a chance the zombie will gain free will once more if you do heal it, with that chance being determined by how much you healed it (rounded up to the nearest 5%). IE: if you healed the zombie by 14HP, then it would have a 15% chance to break from the necromancer's control.