Assistant is a toolbox app that goes on your PC or Mac. I've lived w/o it because on the Mac it doesn't talk to the bird. Though someone has suggested I power up the bird before launching the app. Will try one of these days.
A gimbal calibration won't hurt. Takes 1 minute.
Joysticks don't need it until they need it and the procedure is a PITA.
Vision system calibration isn't needed until it's needed. After a hard crash is it advised.
Some prefer doing the firmware updates with assistant.
Don't calibrate IMU or compass needlessly.