Is it proven that more up time on body vs the % per leg killed is better? I know legs are mainly for the kiters too so that they have more time, so maybe only worry about killing 2 legs? *shrug*
It is proven that getting the full value from those kinds of thing are better yes, because they give you the most for the cost.
It's the same kind of logic when, say, Lich King Raging Spirits were being killed; people would switch to the new one when the old one was at 20-30% and let dots tick it down to maximise their damage. If you burst a Leg down while dots are ticking on it, all those dots haven't really gotten their benefit out, but you've already sunk the cost by casting them.