Actually as long as you use a mild shampoo that doesn't strip the coat of protective oils, there's nothing wrong with washing a dog as often as it's needed.
The whole claim that dogs shouldn't be given a bath more often than once a month or even less (or it ruins their coat, blah, blah, blah) is just as silly as when people used to think that they shouldn't shower/bathe regularly because it's "unhealthy".
Shampoos made with natural detergents (e.g. coconut shampoo base etc.) and without harsh chemical ingredients can be used frequently and will not disrupt the skin balance in any way, unlike harsh soaps and shampoos you find at mass retailers and department stores. True skin nourishment comes from the inside anyway.
One thing a lot of people also don't take into consideration is that being "dirty" today has a completely different meaning than for example 50 years ago. There is so much more pollution now, both from above (air, rain) and below (car tires abrading on asphalt, dust binding pollutants and settling etc.) and dogs generally are lower to the ground than people, they can't change their clothes every day, so they are much more exposed to it.
Taking my own dog for example, he currently gets a bath once a week since both his obedience and agility classes are at a public park in a huge city, in areas where other dogs pee and poop, it's hot, and there's no denying he picks up a yucky smell from that. I rinse his feet with plain water every time after we exercise (walking, biking, rollerblading), but on Thursdays after agility class I shampoo him down.
His skin has truly benefitted from this and the frequency certainly doesn't harm. The key is finding a product that works, which might take a little experimenting. I've been through ones that were ok on the skin but left the coat too "oily", some that made the coat nice but were too drying on the skin and so on.
My personal favorite products are "Earthbath" (especially the
oatmeal and aloe, which has a really yummy vanilla scent) and Buddy Wash by Cloudstar.
