Paul, you should be using Campden (or the unadulterated potassium
metabisulfite) during the entire winemaking process. Apple juice
sucks up oxygen readily, so protecting it at the outset is very
important.

Still, protecting it before bottling is better than not protecting it
at all. But I would add the Campden to the bulk wine and then bottle
it. The tablet needs to be crushed very finely (like flour) and then
thoroughly dissolved in the wine. This takes a lot of stirring (best
to dissolve the powder in a little warm water, do most of the stirring
there, and add that to the wine) and you would only want to do that
once (in bulk) as opposed to doing it X number of times for X bottles
of wine.

Jack Keller, "The Winemaking Home Page"
http://winemaking.jackkeller.net/