This is a little unrelated to this thread, but I'd be REALLY cautious of buying any fish from Petco. Long story short, they don't take care of their fish the way they should. Ask the nearly dead male Betta my girlfriend rescued from there yesterday... She's a lurker on here, but doesn't post much. My two female bettas I rescued weren't a ton better. After yesterday, I'm done.
OK, soapbox rant over with...
Back to the topic, I'm new here, but not to fishkeeping. I agree that salt isnt needed unless you are medicating/filling your tank with distilled water.