I know I have mentioned this before, but the dji firmware beta engineer's said it should be done before each start and flight. It came up when they were asked why most of the time the calibrate compass appears in the app menu even if the a/c had been calibrated. And before thry responded another person asked if it needed to be done every flight, or not. Their response was what I mentioned in the 1st sentence. They were asked why it does not say to do that in the manual or any place else, and that it would be nice if it did. And......like other stuff they never responded back.