You do want to use Impact when its up, but usually I save it for an instance where I dont have a stone or fire available. So I may go Verfire to Aero, then Stone to thunder, but fire didnt proc, so then its Impact to thunder and if my black mana is getting to high, like more than a 15 point lead, then I can do a stone to Aero and even them up. I am probably not maxing out my dps this way or I am making it more complicated then it needs to be, but I usually don't end up with a gross imbalance around my melee combo time and from the 24 man parses I got, my dps is pretty well.