Depends. If I'm going for speed and conditions are right (tailwind, sufficiently steep downhill, pavement not too choppy), I'll tuck and set the feet at 3 and 9 o' clock, knees in, etc. There's one particular downhill along one of my usual routes where coasting like that is as fast as, or faster than, my best efforts pedaling with 52/13 maximum gear.
Otherwise I seem to favor having one foot down at 6 'o clock, usually the right (?)... or is it the left? Now I'll need to go ride again to be sure which.