Just as with the NEC, our code in Britain does not address the issue of 3-way switch orientation, but I always wire them so that "both up" and "both down" are off (of course, our regular light switches here are up=off, down=on, but for 3-ways it works out the same on both sides of the Atlantic).
Where a 4-way is involved as well, I wire so that all switches up = off.
