I've been feeding my dogs, friends dogs, mom's dogs spent grains for close to year. I asked my vet and he didn't see anything bad about it. The only thing I have found is that they do help make the dog "regular".
Recent studies show that dogs are very susceptible to grain intolerance. They'll never tell you because they love the taste but if fed a high grain diet some dogs can develop things like skin allergies.
If you loom at dog food trends they are going more protein focused which is proper for animals of the canine genus.