Joey has grade 1 in one leg, and his breeder suggested I give him glucosamine to keep it from getting any worse. I'm not sure if there's any real proof of this, but it does keep the joints lubricated. I take it and it makes a major difference for me.
He's almost 6 and I've been giving it to him about 5 years, and it's still at grade one, so maybe it is working. From what I've read, it can't hurt.