Just before shooting. I'll patch out the chamber and bore with denatured alcohol, run a patch or two w/Lock-Ease through the bore(keep it out of the chamber), allow a few minutes for the carrier to evaporate then have at it.
Typically the first shot closer to the velocity and group of the following shots in the string. It can also help to reduce copper fouling.
Some say its bunk, but when guys like Tony Boyer and Speedy recommend and use it I guess its good'nuf for me and my rusty old clubs.