Spontaneous combustion from oily rags comes through the build-up of heat through the oxidation of the oils in them. Dried or cured oily rags present a substantially diminished danger. So long as your oily rag pile is somewhat open (and not in a closed container) the heat generated from oxidation will have a chance to dissipate. On a gun cured stock most of the oil has already oxidized (if the finish is hard) so the risk of the gun starting a fire in the box (which is pretty big anyway) is extremely low.
