Paleryder, The answer is probably do it either way, but just backing the flat portion would be easier. My question to you...
Why would you do both? Trapping is done to reduce the strength of the back, so as not to overpower a belly made of weaker wood. Backing with rawhide is done specifically to keep the back from lifting a splinter, or otherwise destroying itself. So if the back is too strong for the belly, why isn't it strong enough to do without rawhide? Ron