Remy started puppy class at 15 weeks. At my vet's recombination, I started taking him to pet stores, Lowe's and Home Depot when he was 12 weeks old, because he had a temper tantrum at 9 1/2 weeks at vet the day after I brought him home, and then then at 11 1/2 weeks he tried to bite the vet and had to be muzzled. I would take hime anywhere they would allows pets, he would ride in the shopping cart and people would pet him etc. The vet felt that the benefits would outweigh the risk.
We had one more incident of him trying to bite some at around 13 weeks, but that was the last time. Between all the outings, going to the vet every two to three weeks because we didn't want to double up on shots, and puppy class -- he has become a very sweet little guy. He is a behavior success story as far as the vet is concerned and I think the exposure to all the different places and people early on really helped. He is 6 1/2 months now and we still have things to work on - but he has come a long way.