In the majority of cases, the fat end of the shoot was the nock end. Native American arrows are barrel tapered with a bulbous or expaning nock. You can't easily carve a bulbous nock from the skinny end of the shoot. Some people like to think that some NA nocks were expanded by splitting the nock and then bending the wood out to form a Y shape. It works, but all the nocks I've seen on NA arrows were carved.
Also, there is a bit of folklore behind NA arrow design. Many believed that an arrow "likes" to be shot with the skinny end pointed toward the target, similar to the way it grows (and points to the sky). The same concept applied to bows: the bow's top end was the end growing toward the sky. Some cultures added hair or facial carvings to the top end of the bow as well.
