Money and Currency for Fantasy Authors

Money makes the world go round, but it gets little attention in some fantasy worlds. Tolkien gives us a few throwaway lines about silver pennies. Modern authors can be much worse, with systems of gold, silver, & copper pieces that have nothing to do with real coinage. Why does it matter? Currency can add flavor and a sense of reality to a world, while subtly cluing the reader in about its society at the same time.