if you are careful about adjustment (a little free play that just disappears as the QR is used to tighten the wheel in the frame is correct; no free play with the wheel out of the frame means you are destroying the bearings) and you use a grease that is heavily loaded with solid lubricants, the cones will usually either not get much worse or may even improve slightly.
You can gauge what is happening by examining the cones carefully under a strong magnifying glass or a low power microscope. Rounded edges to any marks in the cone surface mean that the marks have been there for many miles and may have stabilised. Jaggy edges to marks and/or bits of metal in the grease mean that the damage is ongoing.
To check for debris, wash the balls down in solvent over a wad of tissues and fish around the residue using a strong magnet. You will soon find any debris that has been freshly generated.
There are some things to watch out for; campag cones vary in profile with the hub model; if you have the wrong cones they will carry on breaking up for ever. If you trash the hub inserts you will feel pretty annoyed with yourself.
The method of manufacture of cones also varies with hub model; Record quality cones appear to be hardened differently from some others. This is important because the better quality cones seem to have a thicker hard layer on the surface and this means they are more resistant to subsurface fatigue damage (very likely if the hubs are adjusted too tightly, i.e. with no free play). Once cheaper cones start to crack up through subsurface fatigue, sometimes they are irrevocably damaged and they just carry on breaking up no matter what you do.
If you take a cone, mount it in an electric drill, and polish it lightly with emery cloth, you will cause any marks on the cone's surface to have a slight radius to the edges. This not only means that you can see whether this is old or new damage the next time you inspect, but it also lessens the chance of further damage arising.
Record quality cones will (IME) take a light grind and this will still leave a hard layer that is thick enough to work OK if you are careful about adjustment. If you have the bearings set too tight (i.e. no free play with the wheel out of the bike) then the reduced hard layer thickness won't be enough for the loads imposed and the cone will break up.
If there is just one damaged area on a cone, you can mark the outside of the cone to show where the damaged area is, and then install the wheel so that the mark (and damage) is uppermost. Any service loading won't pass though the damaged part of the cone surface so it should not cause further wear. However if the bearings are not correctly adjusted, you can still get further wear occurring.
cheers
