these days alot of healers don't even target -- mouseover macros and healbot for ez mode. If you die in a 5-man cuz the healer couldn't target you with the boss between you, it really is the healer's fault... They have 10 additional yards of range over their damage spells to allow the boss to be closer to them than you are. That's not to say that 100% of the time you want the boss between you and the healer (I'm thinking waaaaay back to Magmadar on this one), but it's generally a safe assumption to start that way until you find out it's a problem.
as far as when to use a mitigation cooldown... best advice I can offer is when you need it. There's no hard-fast rule of "when you drop below this" or "when the boss does that." Just figure if you know you're about to get slammed, hit it. If your healer just got silenced, be ready to hit it. With the cooldowns short enough that you can wait them out for a pull or sometimes even hit them twice, error on the side of caution and make sure you live long enough to think back "I guess I didn't need to do that."
and finally: in the game you need armor. At the keyboard, you need thick skin. You're going to be blamed, whether it's your fault or not. Be mature about it, and make sure you're doing the best job you can do. It'll bring back the ones worth grouping with again.
I'm a tank. I like the beatings.