If you have a rear wheel carrier that holds spares behind the rear barn doors or tailgate, do you cover your spare(s) or just leave them uncovered? Given that the spare(s) are going to be getting bulk sun-time and never rotate on the spare wheel carrier(s), what's the likelihood that spare(s) will get excessive UV degradation to the rubber if they are not used or only changed out with a full 5 or 6 wheel rotation service pattern?