Set it to 'neutral', that way only the black and white points are set, and if you go to the 'image' menu and then click on 'graph B/W' a histogram will come up which allows you to set the black and white points using the two markers on it. Turn 'pixel colours' (under the 'colour' tab) on so that you can see if anything is clipping when you adjust the points, try to set them so that nothing is clipping as that way you'll have the most information available from the scan. You'll probably also want to add in some contrast as well (its better to this afterwards in Photoshop though in my experience).
However I usually just tend to set it to 'none', and then just do all of the above in Photoshop etc by setting the levels and then adding in some contrast as it gives me much more control over the final result.