[Dalnaresh]

In instances I'd stick a low rank of corruption (nightfall ftw!) on secondary targets and then get down to spamming shadowbolts on your nuke target. A single thunderclap and demo shout will mean you won't pull aggro unless the tank gets two resists in a row (unlikely). If you see a caster mob, stick CoT on it once the tank has established threat. Play the whole thing by ear. You never know when you might need to CC something in an emergancy - a dot isn't going to allow that.

In raids its slightly different. Unless you have a specific task to do (Curse of Tongues on a specific add, Banish duty) do what you are most comfortable with. Personally I don't like doing more than dpsing down the kill order one by one (you never know when a lag spike or something takes out an offtank and I really don't want a mob heading to me because I've been dotting it for the last 20-30 seconds...). Best case I'll stick a corruption up in order to increase my nightfall proc potential on my main target.

If you are on banish duty, I'd be tempted in most cases (especially in the Void Reaver chamber) to do almost nothing except banish. Keeping these mobs locked down until a tank is ready to pick them up is more important than my dps in these situations, and the same is usually true for our mages on sheep duty.