As a recent puppy owner I'm new to what is considered good manners when in the presence of other dogs and their owners. But I'm going to go out on a limb here and say if a person (me, for example) walks into a vet office with a crying toddler on her left hip (let's say because a puppy recently licked his toes and he didn't like it) and a leash attached to a crazy puppy in her right hand don't let your massive St. Bernard dog approach them. And don't think the phrase "he's really friendly" will make it okay. I could care less if he's friendly because 1) he's still getting my already hyper dog to act even crazier and 2) he's an animal - you don't really know every move he's going to make. I saw the movie Beethoven, I'm no fool as to the havoc a large dog can create.
And for the record I have a puppy not because I'm an animal lover, but rather because I'm a very nice wife.