C1 C2 & C3?
I think we will all use C1, C2 & C3 very differently according to our profession. When I'm shooting runways at fashion weeks I have C1 and C2 set to baselines for the various tents/halls. Usually there is one big tent (call it C1) and one or two smaller venues (C2). Because lights change from show to show it is necessary to make adjustments of course, but it is really nice to have a baseline set (AF Servo - mode 5, ISO1250, f6.3, 1/200, RAW+S, highspeed drive, WB=3700K, focus points, etc etc). My C3 is for backstage beauty shooting (one shot, single shot, ISO800, tungsten, center micro focus point)
I don't use auto update. I like having a known baseline and then make changes to this. If I decide to make a new baseline then I'll do that manually and save over the previous C1...
Finally, the 2nd multifunction button (next to the DoF preview button on my 1DX) is reprogrammed to be a toggle between AF Servo and One Shot. This is awesome for getting the pose (and reframing) when the model stops at the end of the runway.The DoF button itself activates the electronic level. There are a bunch more customizations I use, but these are the key ones.
But as I said, these are appropriate to my shooting needs, I'm sure landscape, studio, sports etc etc shooters will differ wildly.