Dogs that have a healthy skin and coat should be bathed as as infrequently as possible. Dogs have a oils in their skin that they need for a healthy skin and coat, every time they get a bath that layer is washed away. Many skin problems dogs have are caused by frequent baths. Every dog is different but as a rule do not bath more then twice a month, unless Fido gets himself into something stinky. Always use a quality shampoo, look for a shampoo that is soap free, these shampoos will have cleansers but not detergents. I like oatmeal based shampoos because I find them to be soothing especially on an allergy dog.
As an alternative to bathing try brushing your dog. This pulls away loose hair and skin flakes. Your dog will look better and feel better.
If your dog has a skin problem such as an infection and your veterinarian has recommended shampoos and bathing then their instructions should be followed. Many shampoos are medicated and are formulated specifically to be used more often in order to treat the animals skin.