I think having a pet in dental school is a good stress reliever. We adopted a one year old Bichon Frise, and its a joy to have her around. I am taking 21 credits right now and I come home exhausted, but after being with her for an hour, all of my stress just melts away.
Mind you, I have a wife who is willing to take care of it while I have long days at college and I know I will have the same routine at the dental school. If I didn't have my wife to take care of her, while I am away during the day, I don't think we would have adapoted her.
Dogs can be demanding, but than again they are also very trainable.