The whole purpose of the campden at the beginning is to keep wild yeasts and other buggers from taking hold before your wine yeast can multiply. Wild yeast has a lower tolerance to sulfur than the actual wine yeast that is selected for this purpose. Adding it later may help protect from oxidation, but is by no means necessarily. In fact you don't need to add anything once the SG stabilizes, and it clears up. You can just bottle it. Unless you plan to add sugar and back sweeten it. Then you'd add about a tablespoon of sorbate at the same time to prevent your bottles from becoming bombs.
