I don't often let a tank dip below 40-50% unless there's minimal damage going out. If they happen to dip to 30% or below it's usually a mistake on my timing or I've already made the decision to use Benediction, so might as well squeeze as much hp as possible out of it. It's not really intentional on my part though; I don't play chicken with a tank's health, I'm just trying to be efficient.