For the newer games that were bought nib, keep the receipts obviously. For the games that were aquired via a trade then they will have to be appraised and have official paperwork....at least this is what my insurance agent told me.
It is true that 99 percent of the public knows nothing about pinball machine value and I think it would be difficult getting 15,000 from your insurance provider if your TOTAN went up in flames.
If you can get a policy for 20 bucks a month from a company that deals specifically with pinball machines then I would probably go that route.