What do you really want the peak to look like? If you like the look of molding and think it will enhance the appearance of the room, then go for it. If you think the molding would look like a "cover up", there are ways to permanently correct your cracking issue. First, and if possible, you need to make sure the trusses (rafters) are not moving excessively with the outside temperature changes. Additional attic bracing may need installed to prevent this. Second, you can purchase (from a box store or drywall specialty store) plastic cornerbead that is designed to be installed in the peak of a cathedral. This cornerbead typically come in strips or rolls and are much wider then "normal" cornerbead (about 4-5" on each side). I've used this product many times to correct the cracking problem you are describing. Never had a call-back for additional cracking.