From my experiences working in hospitals:

We would take it whenever we could get it because people are sick, and we didn't want to disturb them too much. We were just concerned with whether or not the readings were within or close to normal range. If we got an abnormal reading, then we would ask the patient to move, or will pull them up and elevate the head of the bed. Sometimes we would have to take BPs on patients' thighs or calfs!!! Those were always off, but I guess the nurse and doctors knew what to expect from readings in those areas.

Also, if we got a reading that was off the charts, or too low, we would take the BP on the other arm. Sometimes if BPs on arms are very different, it's a bad sign, and something that needed to be reported ASAP.