Feel free to repose as often as you like. Once a mob is slept, it doesn't gain aggro unless dots are placed on it before applying sleep, or an enmity move like flash or provoke is used on it. This means that the tank has one less target to cycle through while using their hate combo since they can simply ignore it until the sleep timer is up. If your party is actively aoe'ing every pack, you're more likely to have your sleeps being broken, and should just stick to healing. Otherwise, repose is a great mitigation tool, and lets you put out plenty of dps without having to worry so much about the amount of incoming damage.