As long as they keep you alive it's all good. I don't really panic regardless if my HP is low or high as a tank. I know that I can work with any HP bar as a healer as long as my incoming damage estimation skills and cooldown timing is accurate. I personally set a 30% HP tolerance threshold as critical and 40% HP as a sign to top off, although not as much if it is just 1 weak mob.

I play like I'm comfortable with my party at all times and don't hassle too much if we do wipe - which is really practically never unless the DPS doesn't do the mechanic or the fight drags on too long.