My clients told me that one day they watched as two men with a four foot spirit level got out of a truck (having driven by several times before) and walked up to the dry stone wall we built four years ago and measured it to see if the coursing was running exactly horizontal. It must have been some sort of bet or dispute they were having and they had to prove who was right.
The wall was built on a hill that rose about three inches every foot. The courses do look level, but their not. They were built leaning slightly downhill. To build the wall exactly level would have made it look too severe. Sometimes you have to go with your eye and what looks right. No big deal, but then again the guys measuring the wall thought it was.