If you try and layout a bow from twisted billets you will most likely get grain run off across the back. There is a way to avoid this. *
My point was initially split the billets out of the log with an axe and wedges, as the splitting will show up if the grain is twisted into a spiral, wavy along the length, or straight. You can usually tell with most trees by looking at the bark whether it will yield straight grain or if its twisted like a barbers pole, despite the log itself being straight.
You can also look at the log with out the bark(but then you have to worry about it checking), If you look closely(you may need a magnifying glass) you can see how the grain runs.
*Once you have found if the grain of the billets is twisted( but at a level that will still yield a bow, then instead of laying out a bow with straight lines, you free hand draw a centreline, that follows the run of the grain. I'm not sure which volume of the bowyers bibles, but there are photo's that explain how to do it. (article by Jim Hamm maybe) I really suggest you get a hold of all 4 volumes.
Hamish.