Frost PvP

I'm a little confused about how/when to Obliterate. I always try to Obliterate when KM is up and only FrS if all my runes are down, but do I Obliterate outside of that? or only on certain situations, like on cloth but not plate.
Obliterate whenever you wanna hit like a truck? Anytime you have runes for it? Whenever you are in melee range? Its your hardest hitting attack, use it accordingly.
i typically save obliterates for any armor value less than plate. you should be HBing any plate class and NSing accordingly.