Lets deal with the first question first, how often should a dog be washed. There's no quick answer to that, because it really depends on the breed of the dog, It also depends on the dogs lifestyle including any health issues they may have.
Here's what I recommend to my clients - most dogs, regardless of breed or size, should be groomed at least once a season - every three months. This will deal with the dead hair and skin that's your dogs been shedding every day for the last 90 days. Even if you brush your dog regularly, there will be a build up of dander and oil from the skin that you wont be able to get with the brush.
However, if your dog is a house pet and sleeps indoors, then you may want to wash your dog more often.
Once or twice a week is perfectly fine to wash your dog - as long as you are using a gentle shampoo - (designed for dogs, not babies,) that is kind to their skin. If you find that their skin is red and itchy, you may want to change the shampoo. I have high quality shampoo for sale in the salon that is Aloe Vera based and very gentle on all skin types.
There is an exception though - curly coated dogs like Bichons and Poodles (including mixes of these breeds) have hair that mats very easily. Unless you are willing to brush your dog more than you wash it, I'm afraid that you'll end up with a matted dog - hair like dreadlocks that pull on their skin and generally make life very uncomfortable. Best leave it to the professionals if this is happening regularly.
- It's too often if the dog is getting matted between grooms. (More on matted dogs another time.)
- It's too often if the shampoo is not suitable for dogs skin and causes irritation.
- If the dog is getting snappy or grumpy at bath time - it could be too much for your dog.
The benefits of grooming your dog regularly, whether you do it yourself, or get it professionally groomed, go further than simply cleaning your dog and having it smell nice. It can also highlight any issues that may be going on that you haven't noticed. For example, when the dog is being washed from tail to nose, you may notice a lump that wasn't there before. You may notice that he won't let you touch a leg, or paw or an ear. An indication that something isn't quite right. In which case you take the dog to the vet and get it checked out. It may just save your pets life!
Remember, this is just my opinion and not to be mistaken for medical or veterinarian advice.
Enjoy your pets!