I'm going to go out on a limb here and suggest that shading rate isn't such a great factor in rendering hair in so far as appearance is concerned (at least in Poser Pro 2010). There is a render time factor.

I'm not a scientist but I have run a few tests. In the test above I couldn't see any discernable visual differences in the second two renders which were both at what I consider to be fairly high and rediculously high shading rates.
The only difference comes with the first one at the shading rate of 0.25, which I usually render most other scene elements at. I can't quite work out whether the horizontal lines that appear are intentional or artifacts.

Notice how, at this very high root width, we can see a jagged structure to the hair strands.
Notice also the effect of the root softening.

 

PoserPro2014(Sr4), Win7 x64, display units set to inches.

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