I did a search to find something about paper money and i'm actually surprised to not find anything. It stands to reason that there would still be a lot of paper money in the environment. We probably won't be using it for currency, but it doe retain some practical use. Paper money could be used to start fires (surely there would be other practical uses for it, but I can't think of any). We could even loot this off of zombies, since I'm sure lots of people died with their wallet in their pocket.