Edit: just saw the post above.
Good question! We've got a Baratheon "star" with a literal seven pointed star on reverse. Such that the image states the denomination. Same with the Aerys and Aegon stags... they have a stag on reverse as the graphic depiction of the denomination. But... we also have house sigils. So Torrhen's "stag" coin does not show a stag, but a wolf... yet the coin by virtue of its size and weight is still a stag. So if the stag is both the name of a denomination AND the sigil of House Baratheon, then it can be confusing to have a Targaryen coin with a (seemingly) Baratheon image on it! Likewise we learn that the main gold denomination is the "dragon", yet the dragon is also the sigil of House Targaryen. So the question revolves around whether it is appropriate to use the dragon as the house sigil on a Targaryen small-coin. I would say yes! Though in that case it would be appropriate to use the textual inscription to make the denomination clear. So for instance a small penny-sized copper for Aegon with a dragon on reverse and inscribed "ONE PENNY" would allay confusion and make perfect sense. Such a piece might require the making of only one die, since the obverse Aegon die would still be applicable.
But in the universe of hundreds of possible coins.... how does one choose which to make? Mostly I have been choosing based entirely upon whim and fancy. For instance the most recent issues for Balon Greyjoy was not based on him being exceptionally central to the story, but just because his sigil was the kraken... and I found that image entrancing and decided to pursue it.
There is another thread in OBJECTS OF ICE AND FIRE: http://asoiaf.wester...-thrones-coins/ wherein I get more detailed about the specifics of different coins. And in one of the posts I offered some options whereby someone might sponsor some particular coin being made, if it is thematically intriguing to them and passes muster canonically.
Be well! Tom Maringer