I've had several NES cartridges that wouldn't play no matter how many times I scrubbed them with alcohol. Only thing that worked was a nice hard pencil eraser.
On a related note, I know when you clean metal with the kind of rubbing alcohol you get at the drug store, it leaves a fine sticky residue that tends to accumulate crap quicker than before. Is that less of an issue with 99% isopropyl alcohol?