I'm do the same as you, one ceiling I'm putting drywall over top of the plaster, I'm going to cut away pieces first to make sure I get a rafter and to have long enough screws. Other ceilings we are going to tape cracks, screw in tar paper tins where it is loose and then skim over. If you are putting new drywall you don't have to skim the whole thing unless you are putting textured ceilings. Good luck