Matt,
not being an authority on spiders I'm not going to tell you how they should be tied but, to me, doubling a hackle lends itself more to a swept back type finish rather than the true -soft, perpendicular, dare I say scraggy result - which would be more desirable for a free drifting river pattern.
The typical, commercial, shop bought 'spiders' have a swept back style - like a traditional wet without a wing (and possibly tail). Dare I say that more river anglers fish spiders down and across (tight line) than upstream dead drift and they are typically pulled on still waters by the majority therefore people are happy to use the mass produced swept back style.
I don't know if this was what you were infact asking but It's my take on things.
I always use scraggy perpendicular hackles for upstream spider fishing.
Iain