Sorry for this scholastic reply.
The looking glass is telling me that wrinkles and puckering occur during life, which for some form of leathers are mechanically removed in the manufacturing process. Then, the result of tanning is crosslinking of broken up collagen fibres, which should confer the leather to be to have homogenous properties with regard to stress. Finally, my foster father had extensive experience from leather making and its use in clothes, and he was very particular on the type of leather, especially for coats and bags.
My “quasi-educated guess” therefore is that factors other than stretch dependent are behind why one should avoid having the spine of a hide visible. Could it have something to do with hair follicles along cattle spines being coarser and deeper, and that having an impact over time?