Even beyond safety and people's opinion's of Alpha rolling, I personally think it would extremely disrespectful to do it to someone else's dog.

I had dominance issues with my pup (since resolved) and I had a couple of friends who felt they were perfectly within their right to discipline my dog when they visited. It angered me greatly when they would walk into my house and correct my dog with their own methods without letting me handle the situation or asking how they should resolve it. I would never, ever use any type of physical reprimands or physical dominance displays with someone else's dog!!!