Gravity (heavy tip) will make a bare shaft fly tail up.
Impact of bare shafts and arrows is how I've tuned for years. I don't pay any attention to nock left, right, up or down. I'm only concerned that bare shafts impact with arrows.
I start at 10 yards and shoot 2 bare shafts and 2 arrows. I get immediate feedback, make adjustments as needed and do it again. Then, move back a few yards and start all over. By the time I get to 25 yards my arrows fly better than I can shoot them.
Interesting that there's three threads about bare shaft tuning on the front page.