I have only ever used tennis balls, and then only for the heavy items (pillows, comforters) that need "fluffing" to dry faster, and wont' dry out side in a day. Otherwise nothing added to dryer when I do a load, line dry weather permitting or for my non-casual clothing.
For the other poster, the dryer sheets add scent (if you like) and reduce static in dryers, and are used in place of fabric softener. I don't like much scent, and rarely have trouble with static, so don't use either. Hmm, maybe I don't put items in the dryer anymore that are prone to static (polyester blends?)