Not only is the sulfite no problem because it gets boiled off later, it actually can help prevent oxidation in the mash tun. I dechlorinate my water with potassium metabisulfite powder, which is cheaper than Campden tablets, and then throw in another 1/4 teaspoon in the mash (ten gallon batch). The amounts are very unscientific. I figure it's cheap insurance against oxidation, even if it isn't really a problem.