Healing is all going to depend on your party setup and the fight or dungeon you're doing. Some tanks you'll be good to throw up a Stoneskin mid-battle, switch to Cleric Stance, and get a 2-3 good Stone II's in before they're even at 75% and you need to switch back. Hell, if you have a really good tank you can even run up to a group of mobs in a dungeon under the above circumstances and throw out 2-3 Holys.

That's not all cases, though. For me, a general rule is I try to keep Stoneskin up on myself and the DPSs at all times, so long as I'm not doing it when I should be curing. It's just that much more added buffer if they eat an AOE or can't/don't dodge an attack. Don't be afraid to Stoneskin a tank mid-battle even if you don't plan on DPSing, that Stoneskin on a tank with 8k HP will give you a buffer of nearly 1500 damage to do other things, like heal up a DPS, remove debuffs, raise someone who may have died, that kind of thing.