Bending for cast would often be easiest, simply because the thinnest dimension of the wrist is usually from side to side. Generally, thinner wood bends easier than thicker wood.
However, there is much more to consider that may factor in, and may explain why some stocks bend easily while others may spring back or even fail by cracking.
Woodworkers find that Black Walnut is (usually) the easiest walnut species to bend. Moisture content matters. Old dry wood does not bend as well as newer wood. Wood that has been damaged by de-oiling with harsh organic solvents is more brittle due to degradation of the lignin and cellulose structure.
The cut of the blank and the grain structure makes a difference, as does the presence of knots or inclusions.
