I go by the Oak Ridge Paper Airplane contest rules...95 percent of the structure should be paper. From my own standpoint, it is not the media that is important, but the media's usefulness for what you are trying to accomplish. Paper (especially after means of digitizing became affordable for the common slob) is the most amenable medium I've run across yet for customizing kits to suit your own fancy, which is very high on my priority list. That said, I have no problem whatever with wire, photoetched brass, thread, cord, chain, map pins, etc being used to depict details that can't be carried off well just with paper. Comes to that, I've been known to substitute food carton foam for cardboard when it is the right thickness for support armatures and formers.