The story I'd heard about photographing in Paris was the whole "using a tripod=professional" thing. If you're not using a tripod, it's definitely not an issue. If you are, just be prepared to A: act the dumb tourist, and B: be able to explain in broken French that you're just an amateur on vacation. Be humble, polite, and apologetic. Dumb American plays much better than Ugly American. I've ended up having some very lovely conversations with policemen who've come up to me to ask what I'm doing when out shooting with a view camera. If you are shooting with a view camera, one of the surest ways to get rid of an overzealous cop who hassles you about what you're doing, not only be enthusiastic in explaining it, but offer to let them take a look under the darkcloth! If they're not a fellow hobbyist, they'll run screaming the other direction and leave you alone. If they are also a photographer, they'll take you up on it and you'll have made a new friend.