Personally I find it nigh impossible to pull aggro as a Frostie unless the tank really sucks. With 3/3 frost channeling and blessing of salvation, that's a 51% threat reduction. You could of course slap on 8/8 Arcanist for further reductions.

So yeah...I usually hit the nuke button after 1 sunder.. 2 sunders if I'm using ZHC