Perhaps a mite far-fetched, but it's not unheard of for fans to become unbalanced. Used to be that just sticking on a label asymmetrically could use the fan to become unbalanced. It's all hydraulics now (on the good fans, of course); but still: I don't think it's s real good idea to blow air from, say, the top right side, directly onto the CPU fan, whist also having your regular, 'linear' flow (from fore to aft). Anyway, probably not the biggest issue.
Motherboard orientation with rear connections using air cooling means front to back airflow is best. Cases with top vents are somewhere between total waste and just plain stupid!. The cause front to back airflow to move upward, which means heated exhaust off of GPU moves up around CPU cooler. Not a good idea.
I think placing 1 top exhaust-fan, to the outermost left position (nearest to the rear exit), is probably okay, flow-wise.
As for GPU, ideally your card takes it all outside the back itself, of course; but, naturally, the card will still get warm.
Ideally we want nothing but front to back airflow with back having as much or more vent area than front. Only real exception is maybe bottom intake flow directly toward GPU intakes and maybe the back top vent helping flow heat air out of case.
1 strategically placed bottom-fan (plus maybe a more-or-less oppositely placed top exhaust fan) might be a consideration too, for when you have a (very hot) 950 Pro SSD or something, that could greatly benefit from having a fan blow over it. That bottom-fan air should, ideally, pass the 950 Pro (mine is seated on PX1 wings, in a PCIex4 slot), and be largely taken in by the video-card intake (typically facing down).
Anyway, thanks for your thoughts on the matter!