Quoted from KozMckPinball:Rules Card or card insert only. Unless it's Brian Eddy or George Gomez, then they can sign wherever they want.
When I was getting signatures at TPF, I went with the service manuals. No one else will see it, of course, but I figured it's just for my own amusement anyway (Steve Ritchie did comment on how flimsy the Star Wars manual seemed while he was signing it, which was pretty funny).