Orientation of the LIGO Arms

The orientation of the interferometer arms at both sites are approximately Northeast-Southwest and Nortwest-Southeast, though I assume that, on account of the Earth's curvature, no pair of arms is exactly parallel. Is there a preferred orientation for the instruments, and is there a benefit in having them in (approximately) the same orientation? My guess on the latter issue is that if an interferometer has blind spots, it is better for those at both sites to be aligned (or at least overlap as much as possible), as a blind spot for one is a blind spot for the instrument as a whole.

For the German GEO 600, I happen to know that the orientation (and the fact that the two arms do not form an angle of $\pi/2$) is mainly due to building site constraints.
• I have just found this: physics.usu.edu/shane/classes/astrophysics/lectures/… . It mentions '+' and 'x' polarization, but to me, the diagram for one looks like the other rotated $\pi/4$. Diagram C seems to be suggesting that an interferometer aligned with its axes would not detect a signal, and if so, this might be a design consideration. I am thinking of asking another question about the angular and polarization sensitivity of an interferometer, which would be a precursor to this one. – sdenham Feb 15 '16 at 14:28