If you want a straight bow, use straight wood. Ash or hickory can usually be found in nice straight-grained pieces, but most other woods don't come like that very often. If you make a straight bow out of a crooked stave, it will likely either break, warp, or twist badly. You can fudge a bit, but the first three rules of selfbow making are (1)follow the grain, (2)follow the grain, and (3)follow the grain. A 14' bandsaw works great for roughing out staves. I do like Pat said, find the centerline, lay out the outline of the bow just cut down the lines. A bow doesn't have to be perfectly square, plumb, level, and straight, you're not building furniture. You're going to have to cut the bow to shape sooner or later, making a perfectly square, straight 2x2 out of the stave just adds an unneccessary step and more work.