To control a process a verifiable baseline is needed. Custom can be set with an Expo Disk, but unless a gray card is also used for an "after" shot there is no objective way to verify the color balance is correct.
My process control outdoors starts with setting the camera to Daylight WB, shooting a gray card, setting WB from that image, then shooting the card again. I know with certainty the second shot is neutral. By toggling back and forth between the Daylight and Custom shots I can tell what the light temp is relative to Daylight. When I edit the files I can open the second shot and measure the card with the eyedropper to verify R=G=B on the card.
Like the ExpoDisk, monitor calibration devices can set RGB gray balance on the monitor, but how to objectively verify it was done correctly? Knowing the post-custom WB card test shot is correct I can use that image as a baseline to confirm my monitor is in fact calibrated correctly. By starting with Custom WB off the card in the camera, the camera color which I can verify the the eyedropper (the most objective quantitative measurement tool I have), not a monitor assumed by not verified neutral, becomes my baseline.
Indoors I usually shoot with dual flash with diffusers, overpowering the ambient. I use the same custom WB procedure from the Daylight setting baseline: my diffusers warm the flash to close to daylight color temp.
When I want to blend flash with tungsten seamlessly I gell the flash with 1/2 CTO and set custom WB off the gelled flash to eliminate the mixed lighting situation.... Shooting the same before / after baseline shots...
Gelling for fluorescent is more problematical because they come in a wide range of color temps. I used a bifurcated strategy of using the ambient only for wide shots and situations where I can't use flash, and flash to overpower the ambient for close-ups. I set Custom WB off the diffused flash in the camera, but also shoot the gray card in the ambient light as an editing reference. That makes the flash shots correct out of the camera, and the fluorescents not. When editing I open the fluorescent test card shot, click balance it, then paste the WB settings to all the other ambient only shots. That makes everything neutral.
Setting Custom WB is less than a minute of work on the front end, but it gives me a neutral baseline for color in every shot I open in the RAW editor as a starting baseline. Neutral WB isn't the goal, consistency is. I may decide to warm or cool the WB based on content or desired mood. Because the WB is consistent on all the shots in the batch I can the copy/paste the edited WB settings into the other files. That's where the time savings is, and the advantage of shooting RAW.
I never use AWB because it resets WB with each shot based on content and there is no consistent baseline shot-to-shot making batch corrections impossible.
