I figure this was a pretty unspoken rule. At the very least, it's something that I've always lived by, and something that our guild upholds to a fairly high standard. And it makes sense: Piss off the healer, and you might not get heals anymore. Do you have a replacement? Then it's time to put something in that cake hole.
Here's my story: I'm on my little paladin, and get recruited for a 10 man ToC pug. Now, these usually end pretty badly. The tanks aren't usually well-geared enough for the encounter, and the healers usually follow suit. But, this is good practice, I figure, for when my healer has to do the real thing for the guild.
So there's this tank that decides to take over as raid leader. And to his credit, he does a decent job at explaining the boss fight in detail. His problem? Not only is he really undergeared (and inappropriately geared, with a 2.6 speed agility weapon and a lot of block rating and block value), but he absolutely has to tell you how -you- wiped the group and how you just need to fucking do better.
First he was all over the melee for not killing the kobolds. Then the ranged for not avoiding the fires. Wipe #6 comes around and he eats a huge spike of damage while he's got 4 or 5 stacks of the bleed debuff on him and I can't keep him up. Next thing you know, in whisper and over vent, he's letting me know that my healing ratios are off, that he knows healing and that I fail.
Now, I'm not the best healer in the world by my own admission. But if someone wants to tell me how to play my class, they better be playing that class at the very least. This guy could barely play his own class.
It didn't help that he was playing a warrior, a class that I know like the back of my hand.
This comes back to my original thought. Never blame the healer. I don't even care if it's their fault. In a guild setting, you have the opportunity to correct any healer issues through other channels, through suggestions from the main healer and other members of the healing ensemble, or through assigned readings from the wealth of articles online. In a pug? Just replace them if you think they're that bad. But don't fucking waste everyone's time trying to micro manage every aspect of a fight. It makes you look like an asshole. That warrior was an asshole.
Why don't you blame the healer? Because you won't get any more heals, which is what happened to this guy. I just left. And ran it later when my guild leader's 10 man had an open spot. Guess what crappy healer got to clear 10 man ToC for the first time?