When I started building wood bows all the books said to pay attention to the grain "feathers" on the belly as an indication of limb thickness, taper and evenness. After working on staves I realized that wasn't the case. If staves were perfect it would matter, but they are not. Now I use my thumb and fingers to feel the thickness of the limbs both lengthwise and side to side. I do use the "feathers" in some cases to see a difference of thickness because the "feather" will sometimes bulge to one side on thicker areas but I go mostly by feel.