Not being a Mason myself, I can't speak to rules or protocol, but from a practical standpoint, I would think that it would depend upon the particular sporran. In essence, if its going to create a noticeable bulge under the apron, I would think that shifting it or foregoing it would be best. If it lies flat against the kilt, and wouldn't interfere with the drape of the apron, then you can leave it be. After all, who's going to look under your apron??? Of course, if there are rules or customs about this thing in your lodge or district, etc., you would of course follow that. Just an outsiders viewpoint.