It depends on the setting. (For those of you playing the drinking game at home, go ahead and take a shot.)
If I'm running a game set in antiquity or a typical D&D sword & sorcery milieu, I'll use gold. In the latter instance, the fact that gold is common enough to be pocket-change for peasants begs for some explanation, and I like to suppose that magic item creation, the casting of powerful spells, petitioning deities for resurrection of the dead, etc. literally consumes gold — alchemically transmutes the gold into baser elements, or sends it into the aether or the heavens — making gold effectively a finite resource in the world, the presence of which roughly correlates with how magical that world is. In some future age, when the gold is rare, so are wizards and dragons and what-not.
If I'm setting a game in a realistic medieval, renaissance, or early modern milieu, I'll use a silver standard up to the 18th century, and a copper standard for 19th century (steampunk, western) or early 20th century (pulp, dieselpunk).
On a tangentially related note, I also tend to be more strict about encumbrance if I'm running a game with a more historically grounded milieu. I'll let sword & sorcery characters have more carrying capacity than in other genres, because, I don't know, the air in a high fantasy world just makes you tougher and stronger for no particular reason — like the Pevensie siblings feeling more vigorous and athletic when they're in Narnia than when they're on Earth.