I think you just have to follow your own heart on this one. Upgrading wood beyond the rest of the gun seems pretty odd. If youa are aiming for an American meat gun, you wouldn't want to stock it with 5x English or Turkish. But then you wouldn't put generic black on a gun that is otherwise a landed gent's gun.
For reasons I don't understand a lot of rifle shooters put very fancy walnut on Sharps rifles - seems completely out of place to me, and I guess what I'm trying to say is keep everything in proportion - whatever that means to you.
Brent