Long ago, I tried to use varied currencies for different realms, with different values and rates of exchange. Like a lot of other posters here, I gave it up. (1) I never had a player who found it interesting. (2) Usually by 2nd or 3rd Level, PC's have found enough loot that it doesn't make a difference.
So I gave each type of coin a flavorful name (platinum imperial, gold crown, silver noble, copper bit) and left it at that.
I occasionally use coins to give hints about a dungeon's origin or history -- "You find a cache of platinum imperials, they are ancient mint and bear the seal of lost Elven King Mrachnor." -- but I don't rely on this trick too often.
So I gave each type of coin a flavorful name (platinum imperial, gold crown, silver noble, copper bit) and left it at that.
I occasionally use coins to give hints about a dungeon's origin or history -- "You find a cache of platinum imperials, they are ancient mint and bear the seal of lost Elven King Mrachnor." -- but I don't rely on this trick too often.