Well - I did a little research of my own from Jack B. Harrison's book - Traditional Bowyer, More Unnecessary Fun...
I found on page 166 and following a section on Riser design and location. Long story short Jack has tried a variety of riser placements as related to the shelf and center of the bow riser. He writes... "I have tried positioning the riser higher, by two inches, lower by two inches, and everywhere in between when building different bows. As long as I compensated for the difference in leverage from the top and bottom lims (tiller), it really didn't seem to make much difference in arrow cast. All the bows shot most arrows properly. Arrow spine and physical arrow weight are of even less concern with the bow center one inch below the arrow rest, so this is the riser position I like."
He explains that with the center of the riser, one inch below the arrowrest it allows him to shoot a variety of differently spined arrows out of the same bow with excellent arrow performance.
FYI...