Mm, for 2h frost, I prioritise obliterate, since that's the point of 2h frost. Since most of the runes are dedicated to a non frost spell, a lot of mastery would be wasted.
It's kind of up to you. Even though haste is better from a pure DPS stand point, I still see a lot of DKs going mastery. You can't think of it as what gives you the most mathematical damage. Mastery is useful because it increases your damage vs heavy armor, which are usually the classes you actually need more damage for. It's pretty much like the old armor pen argument for warriors during mid-wrath.