Yes, it's better to match your billets as the come out of the log......sister billets. You flip one around and then splice them together. Whenever I'm splitting billets out of a log, I number the pairs so I can keep them together. The reason is so that the wood in each limb will match as well as possible. That will make a better bow and be easier to tiller. If you have a bunch of mismatched billets, I would certainly try to match the grain and thickness of the growth rings up the best I could.
Volume I, Chapter 9 of the Traditional Bowyers Bible is about splicing. Several patterns are shown and explained. I like the Z-splice because the splice is identical for both billets. Keep a good, sharp band saw blade and cut them as accurately as you possibly can. I also make sure I have a flat cut on the belly side of the billet that is parallel to the back. This will help you keep the billet flat on the band saw table and assist with a more accurate cut.
Good luck and have fun. You can utilize pieces of wood and splice them in the handle that would otherwise be worthless for a bow.
