Rend sucks, don't bother. If you really want to use it, lay it on the mob as soon as possible to get the most out of the DoT, because otherwise you're just wasting rage if the mob dies before Rend expires. I'd just stick with Heroic Strike as your primary damage ability, it does decent damage and has added threat, so if your Shaman gets some nice crits your Warrior can keep the mob on him.