It depends on how regularly you bathe your dog, but it's better to use dog shampoo. Dog's skin is more sensitive than ours and human shampoo is likely to dry our their skin.
A great question!
My general rule is that human shampoos are often too harsh and excessively fragranced for constant use in dogs. A dog should only be washed at most fortnightly (unless your vet has prescribed a medicated shampoo to be used more frequently). Dog shampoos containing oatmeal are often soothing for dry, itchy coats. Steer clear of the ears and eyes, water in the ear canal leads to ear infections and shampoo in the eyes can cause painful corneal ulcers. Use a damp face washer to clean the face instead. Don't forget to condition the body afterwards to add moisture to the skin and coat and provide a nice shine. Lastly always make sure that your dog is properly dry, especially between toes and in skin folds to prevent infection in these areas.