IMC there are multiple coinages but a few of them are accepted as "universal" due to their well known quality controls and the high detail on the minting. This is similar to the way that the US dollar and the Euro are both widely accepted even in regions that have their own currency.
The party is aware that some coin conversion goes on at times, particularly when dealing with old treasures containing coins too old for people to be sure of the value or authenticity. Some areas frown on the use of foreign currency and have signs posted that an item will cost "1gp-local, 1.15gp-foreign" Since the bulk usery rate is about 10%, it is cheaper to use the money changers.
This is a point of realism I, and my players, enjoy but I reintroduced simplicity in the form of letters of credit. These documents are from well known individuals/groups that usually have local factors, most commonly major nobility, regional merchant guilds, or large organized churches. The local factors have access to low level magics (augery) that will help confirm the validity of the documents and their bearers. Rather than specifying an amount of currency, the letters instead use quantities of commodities ("one pound, four ounces, three drams gold") to be universal.
My players have allied themselves with one of the largest shipping consortiums on the planet and are almost guaranteed access to their funds in any major city. When venturing farther abroad, or in regions where the consortium's presence is thin, they will acquire local letters of credit. Normally letters of credit have a 3-5% surcharge on them (still cheaper than a money changer) but the party has invested heavily in the consortium so they are actually taking loans out against their own investments; reducing profit and increasing liability.
This all fades into the background for the most part. The rogue, just to be safe, carries a handful of jems in his possum pouch.