Before I fly my Mavic Pro I always calibrate the IMU and then the Compass.  Is it necessary to calibrate the IMU each time before flight?

All you do is risk getting a bad calibration and having issues because of it, as said you only need to calibrate the IMU if it flies oddly or after a crash, as for the compass DJI recommend only calibrating if you are flying a distance from where you last flew, with that said ive flown in the UK and then in Portugal without the need to calibrate the compass.

Thank you for clearing up my concern.

Not at all. Calibrate it after unboxing the drone and updating the firmware for the first time then no need to calibrate it again unless the app tells you to or you experience abnormal flight such as the drone not flying straight.

After initial calibration of IMU and compass on acquiring a drone - and I've had several - I've only once found it necessary to do a re-calibration. And that was after a crash. So, no. Set it once and leave it, no matter how far one flight is from the next.

NO!  It is not necessary, or even desirable.  Once properly calibrated, leave it alone unless you have a crash//hard landing/etc.

I did a calibration of compass and MU after unboxing and activation on the P3S and my MPP that was it. That was the initial calibration,not more.

Hello there balans007. Thank you for reaching out and for these information you have given. In addition to this thread it is recommended to calibrate the compass in any of the following situations when flying your DJI Mavic Pro outdoors : First, when flying at a location father than 10 km away from the last flight location. Second, the drone hasn't been flown  for more than one month. Third, a compass interference warning appears in the DJI Go 4 application, and the warning persist after changing location. Thank you.