There's a time to DPS, and a time to heal. I people aren't taking much damage/that damage is easily managed with HoTs, I DPS. When the damage starts to get heavier, I start healing. I use my best judgment and base it off how the other healer is handling things. (Of course I do mess up/lag out when switching Cleric stance on/off sometimes and it doesn't switch off. :'D;;; /stillnoob )
Also, SCH is kind of a "pseudo-DPS" with a healer pet... or at least, that's how I play mine. If I'm on my AST/WHM with a Nocturnal AST or SCH partner, I usually expect to be doing the majority of the raid healing while they focus on DPSing and tending to the tank(s)+throwing some shields on the party when necessary, and vice versa if I'm the Nocturnal AST/SCH. That being said, if the other healer is struggling to keep the raid healthy and/or it's a big damage phase, the other healer needs to start healing. >> Extra DPS is great and all, but like I said... there's a time to DPS, and a time to heal. It should be common sense when to do which.
