Unless the rifle is some sort of benchrest-grade sooperultrasniper rifle that is expected to shoot 0.10 MOA, and you're going to reload the same case for every shot after neck-sizing only, using bullets checked for concentricity, weight-segregated by the hundredth of a grain, etc., etc., AND you're a 0.10 MOA rifleman, it really doesn't matter.
Do it if you want to, but for the overwhelming majority of us, it won't make a bit of difference.
It can be done on a barrel that has a few rounds through it, but is best done on a brand-spankin' new one. The basic concept is to burnish the bore smooth by shooting, while thoroughly cleaning between shots. It's a long and tedious process for a possible tiny increase in accuracy, somewhat less copper fouling, and a bit easier cleaning.