As with collecting all things, there is always the possibility that you may come across an extremely rare and valuable certificate at a garage sale price. Early issues of a company's stock would most likely be the rarest and most valuable. Stocks issued by historically significant, defunct or extremely obscure automobile companies. Signatures of famous persons would make the certificate more valuable even if that person is not directly associated with the auto company. For example, Abraham Lincoln's signature would increase the stock certificate's value even though this Lincoln had nothing to do with the auto company.
Many stock certificates are collectible just for their intrinsic beauty or interesting engraving.
As with the collecting of anything, this should be something you like for your own personal reasons. For example, you may own a Flint automobile or you may have always wanted to own 100 shares of stock in an automotive company. Whatever the reason, it needs to be yours and not anyone else's. This way if you never make a nickel from you investment, it will still be something you enjoy. Stock certificates are often matted and frame-mounted for display as art in a den, study or office.