Under most sun-lighted or sky-lighted conditions, AUTO WB is normally reliable...within reason...sometimes when the light levels are low, like in open shade or in deep shade late in the day, some cameras can go grossly out of whack on white balance,and produce absolutely garish,horrific,cartoonish-like results....the Fuji S2 Pro was terrible at that late in the day as evening came...AUTO WB would be good,good,good, and then BAMMO!!! Absolutely cartoonish,horrible,GARBAGE color...almost totally unusable. With that camera I learned to take a Custom WB about 2.5 hrs before sunset, using a white-painted chair,and I would store that WB for days and days on end, re-setting it throughout the year,winter,spring,summer,fall.
I dunno....AUTO is okay most of the time, but you can also shoot to a pre-set, for greater consistency,and less shot-to-shot variation. Depends. I have used white washcloths....easy to transport in the camera bag, light,cheap, no problems with glares or reflection in-studio like there can be with a gray card and a softbox.
I have also had great success using a telephoto lens, zooming tight, and throwing the image out of focus, and taking a Custom WB off of white or gray clouds in the sky above the scene: this works a LOT like the Expo Disc does,and incorporates a HUGE amount of the incident light that will actually light up your scenes.
During stormy weather, sunrises,"weird light" conditions" sunsets,fireworks, I try to never,ever use AUTO WB, because it neutralizes the odd,beautiful color of the light--always try and use a pre-set under those situations. As always, YMMV, not all cameras do AUTO WB as well as other cams.