- undergeared tanks
Heal as normal. Very likely will have to resort to GCD heals to keep alive. Do so. Tight DPS windows. Do so only when safe, which is usually when the enemy is casting something, but this will likely give you a much needed breather. If the tank still dies after resorting to GCD heal spam; you'll have to let them know that the pull is too large for the group.

- jittery tanks
I'm not exactly sure what you mean by jittery. I'm guessing take a little bit to hit the gas pedal? Just be patient if this is the case.

- selfish (is that the term for that?) tanks
Just roll with the punches. Raising a stink about the situation makes you just as bad.

- a combination of the above
A combination of the above

- My gear is decent enough, so I can outheal most stuff. And a SC Raise is always possible. But y'know how it goes sometimes...
Just keep it up. Over time, you will learn to quickly gauge how much punishment your tanks are able to take. All in all, do your best to support them. There are very few, if any situations where you don't do this.