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Mark, last year, when Scott Kelby gave a presentation, he mentioned the clarity slider several times and suggested that you can push it all the way to the right without detriment. I'm not sure that I agree completely with that approach, but it seems that you can go 65to 70% without the contrast going over the edge. Once again, it depends upon the subject. Jay's answer should be interesting, as he has this post processing thing nailed.
My main concern with pushing the clarity slider is that you start to introduce haloing around the edges, especially when dealing with an aircraft against a blue sky.