I concur with Eric. Establish the limb thickness and taper by gauging it along all four of the limbs edges... following the undulations there... then remove wood on the belly to effect the bend as needed. In case it needs said, the back should first be brought to the pristine growth ring that comprises the bow's back AND it should be brought to an accurate width profile... especially if the belly will be radiused at all.
This is exactly what I've been doing today... on an osage stave with undulations and such. I'm anal about thickness tapering and think it makes things better and easier later, so I really take my time and go over and over it.