The warmth of the puppies is very important. For the first few days they cannot regulate their own body temperatures so they are very susceptible to cold, chills and drafts. Providing or not providing a heat source will depend on your individual environment and relies a lot on your own common sense and observation of the bitch and her pups. Do not make the mistake of keeping the whelping box too hot. If Mom is panting, or seems dehydrated, decrease the heat within the whelping box; or if pups are sprawled out all over the box; you probably have the box to warm. Mom should be able to be in the box, with the pups snuggled against her without panting.
Some people use heat lamps or heating pads and others just keep the heat in the room at a consistent 75 degrees for the first few days. If you use a heat pad it should be under a blanket. Do not put the puppies directly on top of the Heat pad. If you use a heat lamp keep it high above the box so you don’t dehydrate the puppies or accidentally burn the mother. The dam needs to be in the box with the pups for long periods of time to allow almost constant nursing. Overheating the box will make mom very uncomfortable, and discourage her from spending the proper amount of time with her pups.