Actually, no, washroom is *not* a common choice for Americans. Bathroom and Restroom are both common, but I'd say bathroom is slightly more popular - depending on the person's preference and the company one is with.
Another choice some people use is the "powder room." I doubt you'd hear this at, say, a department store, but it would be more common as a guest in someone's home. (And, I have no proof of this, but my gut says it's more of a "high class" thing.) (My friends, for example, would never ask me where the powder room is. :p )
Bethy
