Usually when I rough out a stave I remove the belly wood one growth ring at a time until I get a slight flex, at this point I stop, and round off all the corners and make sure both sides of the limb are the same thickness and start to build my tapers. . Ideally I will end up with nice litte V's pointing toward the tip after this stage.
It always puzzles me how the growth rings on one side of the bow can end up almost a 1/4" thicker than the rings on the other side when the are only about 1 1/2" apart. I don't run into this often but it is not all that unusual either. My quess is that even though the sides of the limbs are cut at 90 degrees to the back it is somehow not how the tree grew.