I have heard that all shafts will have a preferred way that they would bow when being compressed. Even spliced shafts and carbon fiber, although probably less than a natural wood shaft. I would think that density, grain and figure would all play a part in that.
I guess putting a shaft between centers and compressing it while turning it with a dial indicator might show you where the spine naturally is. Maybe repeating this procedure with many shafts you might be able to see repeated results in the direction that wood shafts prefer to bow due to the position of the grain?