So basically, the early idea of currency is that it's an extension of the barter economy with a universal middle-man good. The hammersmith needs bread, and he's got hammers. The baker has bread, but doesn't need hammers. The hammersmith might be able to trade his hammers for fish or flour or tar or something that the baker actually needs and then trade that to the baker, but he's a hammer smith, he doesn't have time for that. So instead, there's a good that basically everybody wants or needs, or at least considers valuable. And thus, the hammersmith actually trades his hammers for gold, and then trades the gold to the baker so he can finally eat some breakfast.
To my knowledge, making a coin with the face of your king or whatever on it didn't add any value. The coin was worth the metal it was made from, and the point of stamping it was basically the king's seal of approval that it was a specific amount of gold so you didn't have to weigh it for every transaction. Bankers would get in trouble for shaving gold from the edges of coins, because that decreased their worth.
The origin of modern paper money comes from bank vouchers or "bank notes". The idea is that your gold is in the bank, and you have a paper receipt that allows you to go to that bank and reclaim your gold at any time. So if you want to use your banked gold to buy bread, instead of going to the bank, withdrawing your gold and bringing it to the bakery, you can just hand your baker the receipt for the gold, so that he can go to the bank and withdraw the gold whenever he wants.
So unless the coin is valued as a work of art, the original post idea that minting them badly devalues them is bunk. Maybe the merchants will want to weigh these coins themselves to be sure, but they are still worth the metal they are made of.