"Drop dead, will you?" I would never use. It's probably equal to telling someone to shut up or go away, but it depends on the context and I suppose it could be said in a joking way. "Drop in sometimes" is an informal way of saying visit occasionally. So if you went on holiday, you might ask your neighbour to drop in sometimes to feed the cat. Or you might say to a friend that they could drop in any time they liked for a cup of tea.