I'm a bit reluctant to post this, but here goes:
It has been my experience of observation, over many, many years of fly fishing that, IN GENERAL, before taking flight, duns tend to float downstream "facing downstream". Granted, many times currents and micro currents change their drift, but that is why I say "In General".
If I my observations are correct, that is why I tend to fish either downstream to fish, or, at the least, sort of across and down in order that the fly will, in its float, be oriented to floating wings first headed downstream.
Agree or disagree?