Well, there's usually a traditional arrow wood each tribe prefers. When we gather any plant material, we don't usually strip the source but gather a little from several sources. So we'd have several spots with a steadily regular supply. And we'd know those sources, for whatever reason produce the best shafts. We also generally gathered shafts from the part of a mountain that faced the rising sun. The way spine was measured was just a simple "bend test". When you know your bow and arrow, you can flex the shaft and know instinctively what's going to be best. The proof was in the shooting. We'd cull the best for hunting. The not-so-hot shooters were separated for battle when dropping arrows down on a massed enemy didn't require such pinpoint accuracy. Or we'd use those for fire arrows for firing a village.
Another thing. We had a "medicine" that told us certain stands of shoots would make good arrows. In a sense, the plant "told" us it would be good. Often, making arrows required certain prayers. Many tribes also had dedicated arrowmakers who would make arrows for trade and he just knew the craft instinctively. Even now, around here, an arrowmaker has a prestige somewhat similar to being a medicine person. And it's also a type of brotherhood. It's hard to divorce the spiritual from the mechanical in Native beliefs and ways. The bow and arrow still holds a lot of spiritual power. If a man is having a ceremony done for him to remove negative influences, an arrowhead will be tied into his hair on his left side because that's the hand he carries his bow in. That's the hand he carries the weapon he defends his people against hunger with. For a woman it is the right side because her weapon is the cooking paddle. We also see that each thing has its own spirit, some call this Manitou in some tribes, and so even an arrow has this. Especially since the arrow came from a plant which has its own spirit. So, if you can communicate with these spirits, you can express your needs to the arrow.
Color has importance, too. We didn't crest our arrows just for decoration. It was tribal affiliation, yes, but each warrior/hunter crested his arrows with his own personal colors and crest which were his medicine. This gives accuracy and power to the arrow. We also grooved our arrows because this represents lightning which is closely related to the arrow. But we also knew it helped keep the arrow from warping.
A bow and arrows were kept in a special place and not hidden away. A quiver might be decorated with personal power designs or amulets and so might the bow. No one was usually asked about this. If he wished to share, he might, but he wouldn't go into great detail about it and what they meant.