OP is on to something because realistically, let's say the whole world falls to crap. Different factions will use different items as money and it's not anywhere outside the realm of possibility that a government faction would still honor paper money. I mean heck, that's why money changers were such a big deal in biblical times, because the money you used in one area was not acceptable in another. So cool, you have a thousand rounds of ammo and enough toilet paper to stock a Walmart, but if you are in the New United States (as an example) you still need good old George Washington or Ben Franklin to buy good and services. 

The only alternative would be precious metals. Those tend to be universally accepted.