Before I even think about DPSing as a WHM, I usually look at how the tank is tanking the dungeon and if they use CDs during large pulls.
If the tank is pulling small, then i will put out all of my regen spells and switch to Cleric Stance.
If the tank is pulling large amounts of mobs, I will put out all regen healing spells, switch to DPS stance and Holy spam, final Swiftcast Aero III until 50% - 40% HP.
I don't like just standing there doing nothing as a WHM myself.