I have always used salt and have never had any problems and that goes with eveyone else I know. Maybe rock salt might harm it...But if your are just using table salt you will be fine!
Just don't smoke around crowds. If your just chilling in a park or something no one will smell it...If someone walks by just play it cool. I doubt anyone will care. If you see someone staring at you or acting funny then I would get outta there. Most people wouldn't even know whats up. Just play...
I wouldn't worry about it. I wouldn't smoke by a school though! Are you just smoking joints? Most people wouldn't even pay attention, Just looks like your smoking a cigarette.