I think that we would use nukes on North Korea, just not on a grand enought scale to completely wipe out the country. We would most likely use them against fixed missile bases and silos, military instalations where troops and combat equiptment is held. That's what I'm trying to say.
Like I said the rest of the exchange would consist of sub-nuclear missiles, basically naval-based Tomahawks. Nukes are strategic weapons. We are not going to sink the country with nukes, we would most likely use them to stop North Korea from retaliating. After that, it is likely that we would invade the North from the DMZ and take Pyongyang.