I have a couple of friends who are considering their first tattoos so i've tried to steer them in the right direction as best i can (recommending shops, loaning them books etc). It was difficult at first to explain why their ideas wouldn't work without coming across as condescending, but i think eventually they'll be happy i took the time to warn them off certain things.
I certainly wouldn't approach anyone in the street to offer a negative opinion of their tattoos, but if asked directly i'd try to give an honest opinion without causing too much offence. If that makes me a snob, then so be it.