Boss auto-attacks are generally pretty trivial and everything else is scripted. If the tank is at ~30% HP (20% is pushing it) but I know there's not a tank buster or some other massive damage for another 6-7 GCDs, Stone IV it is. For WHM specifically, compulsively topping off HP bars is a complete waste of one of your core strengths. Let Regen do work.

Btw, I love how this conversation dominates literally every thread in this subforum, but somehow discussions never wind up being about all the tanks who just eat Vuln stacks for days and what a drag they are on my healer DPS. Why is that?