Well, to answer my own question in case it helps somebody at a later date. In the Traditional Archer's Bible, Volume 1, page 315-316, in a very interesting chapter on arrow making written by a fellow named Jay Massey it says:
Nocks cut from a separate piece of hardwood or horn and glued onto the shaft add a beautiful touch to a handmade arrow, but they aren't at all necessary if your shaft has a diameter of 11/32 inch or greater at the nock end. This is true even when you're using a softer wood such as cedar, spruce or fir. ...... I have found self nocks to be sufficiently strong even without such reinforcing for bows ranging up to 75 pounds. During the years I've used self nocks, I've broken only two even though I have never reinforced the wood in any way.... meaning that you want to start adding horn inserts or nocks to your arrows once the draw weight enters the 75-90 pound range. Furthermore, the poundage you want to start doing that at probably also varies depending on the type of wood you are using. Since my arrow shafts are made from pine which is one of the softer types of wood and my bows are below 75 pounds in draw weight I'm going to take his word for it but maybe with the added precaution of adding a reinforcing binding just below the nock. A little paranoia never hurt anybody.
