The amount of bad advice here is impressive. Just because you got away without re-calibrating when you went on your big trip, does NOT make it advisable. I'd suggest first understanding why and what compass calibration does before you open your gob and insinuate that calibration is somehow unimportant.
DJI advises you do it when you make changes to your bird and when you travel over a reasonable distance. I would stick with these at a minimum. As to when during the power up cycle is best, I don't think there is a clear answer. One could argue that once the Phantom has finished acquiring satellites and locked in course and position, it will be less likely to have something interfere with the calibration given the Naza isn't exactly 100% error free.
I do recall reading in one or more DJI docs (don't recall which) that their procedure was to wait until the FC was ready to fly and then calibrate, power down and restart.