In general, following a breast reduction the breasts are swollen and there may be several areas that are numb. Placing heat on these areas may result in blistering as you may not be able to adequately determine if the temperature still hot. In addition I heating pad will cause further swelling because it will help the soft tissues dilate. I would advise simply to let the body tend to heal itself and to put up with some of the swelling which will inevitably go away on its own within the first 7-10 days. Of course, your plastic surgeon will be able to direct to more specifically.