I use them only when the must is first mixed and 2 days before the first
racking. I use the equivalent of two campden tablets per gallon.
I have found it beneficial to put them a day or two before the first racking
as it seems to make the wine clear.
I wouldn't think it would be necessary at bottling as you have a high
*** level by that point.
On general usage: many books say to use one Campden per gallon ever
2nd racking, though some say every racking. IMO, even one every 2nd
racking is excessive. For more try reading
http://members.tripod.com/~BRotter/SO2.htm, particularly the "Campden