Pretty sure everything worth saying has already been said, but I'm still gonna back up the point that there's no harm in casting Stoneskin, especially when heals aren't required and you've got some spare cast-time. I also understand that one of the arguments against casting it on DPS is that they have low HP so it isn't as effective on them than on a tank, which is ridiculous anyway, considering that if they have a low maximum HP, surely any boost to that is worthwhile...