When should WHM use Stoneskin I?

[VanilleFang]

Bosses yes, swift+stoneskin 2 when you can. Trash mob pulls no, I'd rather have swift for holy stun goodness. Tank can play their role and provoke off a stray on a dps/healer.

This.

If the tank is doing their job, and the DPS are dodging bad stuff, then you generally don't even need to apply SS on the DPS between mobs (Stone Vigil HM is an example of an exception with those dragons using that undodgable AoE).