Usually up for on, but it's hard to distinguish, so most electricians put them in as they want. Sometimes even the labeling on the switch is top-down, as the one in my room. I personally prefer the American way.
Historic facts: Until the mid 70 ies there was the old color coding. Hot was black, neutral grey, and, important for anyone used to the British system, GROUND is red. It's still Legal in old systems and as the wires are mostly in decent shape you'll find it quite often. However, no one stuck to this. Most commonly all wires had the same color. I've seen both wires black, both white, both green, both yellow, zip cord buried in plaster, ...
