I don't use Brasso, but I do use another ammonia based metal polish. Yes, the contacts look slightly more silver, but I'm not too worried about it. Most of the games I picked up over the recent years have been dirty, and generally uncared for. Every game that gets the metal polish treatment will live out the rest of its days in a case.
I was getting to the point where erasers and alcohol just wasn't doing the trick any more. Or, I'd have to do every other time I put a cartridge in. Ever since I started polishing, they work the first time and every time. That's good enough for me.