What is the purpose of the cock feather? I want to fletch my arrows with all white vanes and I have uni-nocks and bushings. I shot a prong still rest so does it matter which vane is down?

I had some all white fletched arrows one time ! The color doesn't matter but it is irretating turning them until you find the right orientation for your rest ! I numbered my arrows on the cock vane and it made it easy to get them right for target practice.............but was totally useless before daylight ......trying to get things set up !!

I went back to a colored cock vane !!!

On a prong rest, the cock vane MUST go between those prongs or you get serious fletching contact and terrible arrow flight.

I like all white fletching too, at least sometimes, but I don't have to worry about twisting and turning the arrow to get the fletching lined up right. I used indexed nocks. When I'm holding the nock and the indexer is under my thumb, then the fletching is oriented just right. If you use plain nocks without an indexer, you can cut a few shallow notches in the nocks with the corner of a file. Orient the nock to the fletching to get proper clearance, and you're good to go.

Just make sure you cut the notches below the level of the nocks' throats. If you make them on the nocks' ears, you can weaken them and they could break.

That is, you can do that if you don't mind using an old trick from the traditional side of archery.

Me too. I shoot all the same color most of the time, or the colors are so close (like flo yellow and white) that I couldn't tell what the cock vane is in the early morning. I go by the nock indexer regardless