In my experience, wine yeast strains are not particularly susceptible to campden. Still, I like to add the campden and then 24 hours later add the yeast. I add campden at every other racking because I don't have an so2 meter. The goal is to try to keep the so2 at 50 ppm, and adding one campden per gallon at every other racking approximates that.
Some people don't use sulfites, and some use far more than I do. I would recommend erring on the side of less sulfites, but still adding enough to protect your wine/mead/cider.