I do brush my dogs teeth, just a few times a week. I don't feel that it's going to far personally. A dog tooth brush and tooth paste is cheap, but having to pay for a vet to do a dental on a dog is not only much much more expensive, but more risky as well (since most vets use anesthesia while preforming dental cleanings on pets).
I use dog tooth paste. Human tooth paste is not safe to use on pets. A tube of good enzymatic dog tooth paste only costs me about $4 and lasts me a pretty long time. It's worth it to me.
It's a myth that feeding hard or kibble foods cleans pets teeth.