I've been watching a friend's dog for about a week and taking it for walks every day. Whenever we encounter another dog on a leash, which is very often, I let the dogs say hi to each other. Dogs say hello by shoving their noses into one another's privates. I just let my dog have his day. Other dog owners seem receptive, but I never know when to say enough-is-enough and break them up. So I leave it to other people. If some kind of etiquette exists among dog owners for preventing their animals from this kind of behavior, then I am not following it.