In Racing, Coning usually reflects two main occurances, wheel sideload distortion, and axle flex. Those tend to wear the wheels towards the inside. Wheels with greater offset in bearing placement (such as Flashbacks or LaCostas) cone more than 2/3rd (Avalon/Cambria) or Centered (Grippin/N-Gen) bearing types. Centered bearing wheels can be, sometimes, flipped to prolong wear, but user beware of traction differential!
Wheels featuring 2/3rds bearing placement wear in nicely indeed if kept on similar axles, and I feel that they gain traction due to the wear (although there's a loss of rolling speed). Not everyone agrees with that, and coning certainly can reduce the wheel's contact patch. Using differing axles can also wear wheels differently, and I think it's likely unwise to swap worn wheels onto differing styles of trucks.
Coning on the outside of wheels is usually slide or kickturn related wear. I say 'usually' as I've seen luge wheels 'scrub' (not Slide) to an outer-coned profile due to high speeds, tempatures, and sideloads, where the wheels have literally stretched laterally until they wear the sidewall/lips of the wheels. Hence, I'm a fan of ABEC 11's Flywheels for Downhilling/Luging, as they're fat, big cored, and centerset.
Trucks that have split axles (Turner TTC) or Offset axles (Tracker RTS Offset, Radikal Dragon's Tail) will flex at the axles a smidge under pumping loads. This affects wheels in how they wear noticably, and that flex, combined with the Grip that true 8mm axles provide will cause distortion especially on rear wheels. Solid axle trucks (Seismic, MMW Trackers) seem to create less cone angle upon the wheels. Stock 5/16" axles on most "over the counter" trucks have inherent slop between the bearings and axle that can make Slalom wheels wear in a 'scalloped' pattern.
To find if your wheels have that 'scalloped' wear in their coning, roll individual wheels on a smooth, level table (I use a draftingboard). If the wheel rolls straight, it's fresh, or at least unmolested. If it rolls in a smooth curve, it's merely coned. If it curves, but not in a smooth curve (or noticably hops), then it's time to either Lathe that wheel, or retire it from Slalom use. I run coned, but still round, wheels often enough to say I feel they're generally O.K. to use, although they may be slower(?) than fresh wheels. I mention Lathing as a means of correcting wear, even though I realize that may be something that is too 'techie' for most. It does extend wheel life, but it usually dosen't make for a faster wheel. Fresh and round are Fast, slightly worn in may be faster, coned, um, I dunno.
Look at Kenny's wheels sometime...
He's a conehead