Dictionary of Problem Words & Expressions (Harry Shaw) says "as if" and "as though" are pretty much interchangeable. Years ago I read in an English usage book that "as if" is preferred for this reason: "She ran out of the house as (she WOULD run) if her hair was on fire" As though doesn't make sense this way. "...though her hair was on fire"? But her hair was not REALLY on fire. Nevertheless, one is used interchangeably with the other.