When do you generally use your hex macro?

Let's say you get someone down to 30% health. I figure at this point in the fight, you could either "waste" a GCD and try get a lucky hex with their healer before using your second GCD for the killing blow, or just use that GCD to throw out another damage spell and hope to finish them off healer or no healer.

The way we've been using it is when the fight is starting to go nowhere and we're in danger of running out of mana before the super important first kill. I.e., they rush in and fear, cyclone, curse of tongues sheep and just CC the hell out of us while a ret pally or warrior goes to town. In those situations it's impossible for us to kill one of them without getting lucky, so we'll trigger the hex macro and hope the gods smile upon us but ti has never contributed anything notable to our chances.