It has been my experience that target materials, and configuration will affect the nock left, right, up, or down.
I have also learned that more important than which way the nock leans in the target, is what the arrow is doing in flight.
I know this can be tough, so hanging a white sheet, or light colored sheet in front of the target butt is a good idea. This will allow for greater visibility of the arrow in flight.
This is a must when bare shaft tuning, because you are tuning for the best arrow attitude while the arrow is in flight. It makes it a little easier when you have a buddy, or two standing behind you. They can focus on the arrow flight, and you can focus on the shot.
If you wait to look at the nock lean in the target butt, you can get bad information.