While I in no way speak as a veteran on the matter, I tend to save it for when something goes wrong, like if you steal Hate or something jumps the healer and the tank can't get over there. That being said, I don't think you're necessarily wrong for Sleeping the non-focused target, but odds are the Tank will be hitting it with AoEs anyhow and generally the Healer shouldn't have trouble keeping the tank up during a single group of enemies. now if a second group gets in on the action, commence the snooze-fest. It should be noted that at L36 you'll get a trait that makes Sleep a targeted AoE, so such control won't take much away from you making boom.