Like Aarron, IMHO having right and regular wood (taking the twist out) makes the task of tillering much easier for the less experienced bowyers. The reason is a twisted limb will not show you the same bending profile from one side to the other. So during the tillering process one must constantly swap sides, to compare the tiller on one side with the other. There are other reasons as well. It definately doesn't HAVE to be done, but one should not discount the notion out of hand.
Than said, some knarly staves will have a little twist even when corrected. And a selfbowyer might as well bump his head on stuff like this, tillering twisted limbs, as he'll be presented w/ projects where options are limited eventually.