I can't give the normal attention I'd normally give to this series of posts tonight, but there will actually be 2-3 posts total today. It's not actually a part of this series of articles, merely a set of house rule that I use in my campaign.
In my world, each nation usually produces its own coins. Additionally each coin is not 100% pure metal. To determine purity, I usually roll 60 + 2d20 to determine the percentage of the valuable metal. Most DMs trying for this level of pseudo-realism usually just quit there. For the longest time, so did I. Then a few years back I began reading a book called "The Year 1000: What Life was like at the Turn of the First Millennium, An Englishman's World" by Robert Lacey and Danny Danziger. The book offers a lot of information, and despite the fact that it's focused on England, it really is a great read for anyone interested in European History from that era.
One interesting thing it brought up was how often that money lost its status as true legal tender. It would seem that in an attempt to stop counterfeiting, the stamp on the coins would be changed every couple of years(in my campaigns I determine the "lifespan" of a coin's status as legal tender with a 1d4+1 year die roll).
What does this mean for a campaign? Well, for starters it means that all that money in a dragon's horde probably has to be converted to legal tender before the PCs can buy anything with it. Doing this requires that the player find and hire the services of a smith with the proper license to make the new coins. Coin changing isn't free however, and PCs can expect to lose a fair percentage of coins to the blacksmith(who is free to charge whatever he'd like) and to the taxes that are invariably placed on this(thus putting more money in the king's pockets). Additionally, just because a coin was made with X% of the precious metal, doesn't mean that it's going to be made with the same percentage again. As such, the player may only end up with less coins than he started out with, and those coins may actually be worth less if he tries to cross the kingdom's borders and has this currency he just had minted changed into the currency of his new kingdom.
For gaming purposes however, a coin only very rarely(5% chance) loses or gains more than 1d6% of the precious metal it's made of.
Un-changed coins aren't completely useless however. One could save the coins and use them to barter with some of the less intelligent humanoids. Old coins can actually be be a priceless treasure all by themselves, assuming a character can find a collector interested in purchasing the coins.