I suppose it depends on what your goal is. If you're creating a building from the inside out, and you're saying to yourself "I want the room to be XXX by YYY by ZZZ units," then you'd do best to hollow negatively, since that preserves those dimensions as the interior wall sizes. If you're more concerned with making sure the whole of the building fits into a slot on your map, you'll probably want to hollow positively, since that will preserve the exterior dimensions.
