We've only bathed them a couple of times a year, using puppy shampoo. Our previous boxers both had skin problems which was helped by diet, but was also helped by limiting their contact with soap and the like.
Having said that, Millie & Henry don't roll in horse poo like Misty used to ! They seem to keep quite clean, unless its a really wet day and the field where they have walks has been churned up by kids on quad bikes.