Giving this more thought, it's always been taught (to me when I started) and I will continue to teach, the accepted standard of keeping the back of a bow "clean", especially board bows. Any deformity will create a weak spot for potential failure. When I'm cutting backing strips for backed board bows, I look for the cleanest possible material, with no knots, swirls, or grain run off.
Actually, when handing over a finished bow to a customer, and especialy with board selfbows, I always tell them to be very careful not to damage the bow, especially the back. Any ding or dent will create a weak spot. So, that being said, I just can't support actually carving into a bow's back. Seems scary to me... just asking for trouble. It looks nice, but not safe. I've (well, actually my artist wife) drawn on the back of plenty of bows, especially those backed with rawhide, with no issues. Some customers want their name or logo carved into the bow, and I always decline. I'd rather not make the bow period, than risk the failure. JMHO.