Back when I was an apprentice sheet metal worker I was hanging some ductwork in an old fire station which was built in the late 1800's. My foreman walked up to me and told me it looked terrible. I put my level on the ductwork and showed him it was indeed level. He had me drop the line until it looked level which was actually 2" out of level. I ended up splitting the difference at 1" out and thats how it's hanging today. The actual building was so out that it made my ductwork look out of level.

My point is that even if something is level it may not look right.