My bet is that perpendicular is optimal because it extends the forward surface area presented by the wheel, lowering the pressure behind it in a more balanced way.
Canting it one way or the other risks creating larger eddy currents than might exist with perpendicular placement. Of course that may or may not be moot depending on how the airflow is behaving around the rear wheel to start with, but I suspect that most vehicles have a fairly balanced, albeit turbulent flow at that point, negating any benefit of directing the airflow one direction or the other.
In other words, like a Gurney Flap, the real goal is to impose a little more energy into the airflow to aid in seperation and reduce the amount imparted into it later.
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