To add to this, I was speaking with another WHM that I group with regularly (heals me in my AK speed runs), and she also feels SS should always be used when the tank is full HP. Basically, if you're between waiting for damage to hit and then casting a heal, or just casting SS while you wait for that damage, you should always stoneskin and stay ahead of the damage intake (as long as MP permits).

In my experience this isn't usually the case and WHM's just wait for that next hit to come so that they can cast cure. Do other WHM's disagree with the above train of thought? If not, why is Stoneskin so rarely used?