To the topic of DC wasteland currency, It seems logical to me that pre-war money would still be a viable currency since it would be impossible to make more. Nevermind the fact that its 200 years later and in our world paper money would have degraded into dust, this is the Fallout world we're talking about and in that world ammo never goes bad, food never spoils, and paper and wood do not degrade. As to who backs the paper money, the caravaners, who are the life blood of many of the DC wasteland's settlements, needing to establish some form of standardized currency to assign monetary value to goods and services decide that the old paper money is what they'll use. Alternatively, they chose bottlecaps because Uncle Roe has a nuka cola addiction.
Thanks for reading my late night rant from a few months ago
My point was that there should be no currency for the DC Wastelands at all, since there are no central banking institutions or authority in the DC Wasteland to back the currency. Economcs in the the post-apocalyptic Westland should regress into pre-feudal trade and barter systems of 'raw' supply and demand where Crazy Wolfgang's scrap metal might be highly valued in Megaton but completely worthless in Rivet City. One of the central function of currency is to peg some consistency to the pricing of goods in order to standardize/facilitate trade. The original Fallout designers had sufficient knowledge in economics to flesh out something resembling a post-war economy. In FO1, the currency for the West Coast Wasteland was bottle caps (it could not be reproduced in post-war environment, thus could not be devalued as a result of 'printing more'). While bottle caps had value in some communities on the West Coast because it was backed by the merchants of The Hub and recongized/used by the communities that traded with them, enclosed vaults and tribal villages that were more or less self-sufficient, did not need/see much trade with other settlements did not recognize them and had little use for them for obvious reasons.
In the DC Wastelands, currency might be used within communities but they would have no value outside of these communities unless what was used as currency were also commodities (eg. if a settlement was using ammunitions as a currency). Logically, the only people who might still use pre-war money as a form of currency would be The Enclave, and possibly The Underworld, the first being an organization that (in some sense) pre-dates the war (since it is a reminant of the US government) and the latter being a community consisting of people who were 'alive' since before the war, and only because it was an inherited practice rather than any kind of belief that paper money has any intrinsic value other than being firewood and toilet paper. For everyone else, it would be more like trading with the Outcasts--you bring them what they want/is in demand by their community, in exchange for what they have in abundance/produce.
As a matter of 'realism', It is more likely that a well-rounded traveller like the player character would have to carry around multiple types of currency, one for Megaton, one for Rivet City, one for The Underworld--all the trading hubs in the DC Wasteland, and one for Dave's Republic (because you know Dave must insist on his own currency...clay carvings made by his kids with his regal visage, maybe), unless these trading hubs had any bilateral or multilateral agreements to standardize their currency (a la the European Union). I don't think anyone else would have any use for currency, other than the travelling caravans of course, because trade is their livelihood, and if anyone was pushing for standardize currency in the DC Wastelands, it would be them,
But as you say, this is a game. The primary function of any sort of trading in this game is so the player can liquidate 'loot.'