I've had a P3P for over a year and a half, and am waiting for my Mavic to be shipped.
Based on the P3P experience with many weekly flights, reading articles and many posts on its forum -- the IMU should be calibrated (1) after each firmware update, (2) if the unit gets badly bumped (hard landing, rough travel), or (3) if it is flying oddly.
It's probably a good idea to calibrate it upon new arrival. It will rarely tell you when it needs it.
But if it's truly flying OK, there's no need to mess with it.
And yes, it needs to be well-chilled before calibration, then calibrated on a level surface that's verified with a level.
There were lots of arguments on the Phantom forum about putting it a refrigerator for 20 minutes before calibrating. That has always worked perfectly for me, because I live in warm climate. Refrigerators are not below freezing and are free of humidity. I place the Phantom in a trash bag while chilling, to avoid slight condensation when it comes out of the fridge. Then remove the bag and calibrate on a level table immediately, before it warms up.
The compass should be calibrated when the unit is flown more than 50-100 miles from its last calibration, or if the app says there are compass errors. Sometimes taking off from a surface with metal (even reinforced concrete) will confuse it.
But on two occasions, I traveled cross-country with it, forgot to calibrate the compass, and it flew just fine.
I suspect that the DJI agent meant to say compass, not IMU. But in reality, nothing needs to be calibrated before every flight.
If everything is working well, leave it alone. If not, do the assorted calibrations.