If you have a good tank, if you bane about 10 seconds in (after your dots, shadowflare, and contagion) you should be okay.

With a bad tank, I don't bane until the 2nd set of dots (or, as usually is the case, the first add is dead).

Shadowflare is a safe dps increase and if the tank can't hold over that he can't hold over the healer either.

Also this entirely depends on whether the healer/BLM sleeps the adds or not.

IF you can get dots in before the sleep, you're golden, as they tick and don't break sleep (I think).