I have a 720E and I do not calibrate it every time. That said, I generally fly from the same place, off the deck of my house. When I start it up I do not get the calibration symbol. If I fly from a different location, I do calibrate.
I have always wondered why the drone needs to be re-calibrated? Both the drone and the phone have GPS. At what point does the drone use the earth's magnetic fields to navigate? Nowhere do I see any control that will move the drone N, S, E, or W and I assume the return to home function follows the GPS back to the starting position.