Am I doing it wrong?

I haven't played much at all since the release of RoTW. I finally got my SSF Sorc decently geared and am starting TZ zones. I don't have a cold sunder (I am currently pure Blizzard). Should I only be doing TZ zones when they're innately non-cold immune?

Once I get one that isn't full of cold immune mobs, per the latest information I saw. The Herald spawns are based on the total % of that zone that are killed, yes?