lol. I used to stop doing this in fights like Turn 2 or King Behemoth where the constant damage to everyone will remove stoneskin in seconds and MedicaII would be a better choice. But then people would yell out, "Stoneskin please!", or the other SCH/WHM/tank will then start casting Stoneskin.
So even if SS will be useless to the fight, I cast it anyway just to give people a sense of security and peace of mind.