Unless you are planning to breed your female dog, it is not a question whether you should spay her or not. Beyond saving you from unwanted litters, spaying your female dog is proven to decrease the chances of developing mammary cancer and other lethal diseases as this article points out.
The question is rather when is the best time to spay your dog? According to the above mentioned article and most veterinarians, the answer is before sexual maturity. It is easy to admit that if your dog is spayed before her first heat cycle (usually 6 months of age), the chances of accidental pregnancy and unwanted litters are zero, which is great, because the problem of overpopulation shouldn't be ignored by any responsible pet owner. So the simple answer may seem to be: the sooner the better.