Depends on the content, healer and what cooldowns i have left.

For Dungeons:
If i play SCH i will Art of War and let the tank drop to 50 % for excog and try to keep him over 50% after that.
As WHM i tend to holy spam and let the tank drop lower. If bene is available, he will drop to around 20% before i hit bene. After that his hp will bounce between 30 and 70 %.

For Trails:
I tend to not let ppl stay on low health, because in pugs ppl usually don´t avoid aoes that good.
As SCH i will shield before AOE dmg and hope my coheal has enough aoe heals (whm op) to bring the grp up.
As WHM i will just press 1 of my 1000 aoe heals after dmg went out to put the grp to 80%+ hp.