I had the same question, but in practice, after two years of flying and experimenting with different seat settings, it hasn't seemed to matter. The cushion just moves the crotch strap forward or after very slightly. I suppose that should there be a force on it, there would be a bit more slack in it if the slot in the cushion isn't directly lined up with the strap bracket, but I doubt it would be enough to make a difference.
RV-9A - Done(ish) 4/5/16! Flying 4/7/16