JREB, here are links to a couple posts I've made concerning that very question. The first link is here on the X-TERRA forum. And the second link takes you to one of my posts on Minelab's TreasureTalk forum. Hope these help you out.
http://www.findmall.com/read.php?55,391759,391799#msg-391799
http://www.minelab.com/usa/treasure-talk/notch-discrimination-on-the-x-terra-ever-wonder-what-you-re-missing-part-2
Part 1 of the Minelab post will explain how using too much discrimination can affect your finds. You might find it interesting as well.
http://www.minelab.com/usa/treasure-talk/notch-discrimination-on-the-x-terra-ever-wonder-what-you-re-missing-part-1
HH Randy