I'm the same height and I encountered this policy on a TA flight from LHR in C last February. I'm infrequent on UA, so I wasn't familiar. Without warning, shortly after settling into my seat an FA approached from behind, slammed the headrest downward, and in a polite tone "reminded" me of the policy. Apparently announcements had been made before I boarded and she was tired of telling people by the time I sat down. When I complained she apologized about the policy and the discomfort of the seat giving me some line about the fact that the plane had been qualified with its headrests in the down position and therefore it was mandatory that they be down for take off and landing. She didn't seem to understand that I wasn't upset about the policy, but rather her brute physical enforcement of it--to the detriment of my neck. But she's there for my safety, right? Blah.