If applying any in-camera sharpening at all (I don't because I shoot RAW) I would keep it on the lowest level for "capture sharpening" only. The problem with in-camera sharpening is that you can never go back so if you sharpen too aggressively in camera you are stuck with the image forever. Also you are then faced with a sharpened image which you may have to process, and in so doing you have put the cart before the horse- sharpening should generally be the last thing you do to an image, not the first. If I shot jpegs only I would turn in-camera sharpening off and sharpen in post-processing.
I would agree with John Chardine's remarks totally. The Nikon D300 is a great camera and so long as you use Nikon's best lenses PS should take care of all your sharpening needs.
The problem with aggressive sharpening in camera (I consider anything over 2 agressive) is that it skews the histogram as well, giving non optimal information to use when setting exposure.
This is why I use a NearUniWB preset with sharpening set to NONE.
I also use a flat tone curve which further allows the generated histogram to approximate a true RAW histogram.