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My thought is, whichever gives the smoothest profile over the tire itself. I would line it up so that it's flush with the plane created by the outermost bulge of tire rubber. This would make me think that mounting it "convex" (bulging out away from the car) would be best in most situations, unless it's a very deep pizza pan, or a very narrow tire that doesn't really go past the edges of the rims. (I would try to avoid a deep pizza pan anyway)
Just my two cents, I'm by no means an aero expert.
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