Really depends on what and where you are shooting. If doing in studio or outside shoots where the light stays about the same for a period of time I would use a Gray card and Custom White Balance. When it changes do a new balance. Takes a few seconds and will give you a neutral balance. Of course you can fine tune it later if you prefer a warmer or cooler look. Of course that only works if you have your exposure set correct to start.
Another issue could be your monitor. It needs to be color balanced. If not you may be shooting a perfect color and it will look orange or blue on screen.
In terms of process control a 8 x 10 gray card is tough to beat. It is big enough to use for Custom WB, and can be used in a reference photo after setting WB to verify it when editing. Over-lens devices can set WB, but can't be used as reference target for checking it or setting it after the fact during editing.
If shooting RAW Custom WB does not affect the actual capture, it adds WB information to the header of the file so the RAW editor will display it neutral when open. Human color perception is so adaptive that there is an advantage to having the first impression of all files come from the same neutral baseline.
The next best approach if Custom WB is not possible is to just include a gray card in a test shot whenever possible. For that task a smaller card can be used. If shooting in RAW the shoot with the card can be corrected with the eye dropper, then those settings can by copied and pasted into all the other files. The net effect is the same as setting Custom WB: the same WB in all the files.
When its not possible to use a gray card it is still desirable from a process control standpoint to keep the WB consistent shot-to-shot. Using the WB preset nearest to the ambient light might not be a perfect match, but all files taken under the same conditions will be off by the same amount. Find one with a neutral tonal area, click to balance it, then copy the settings to the other files similar to when putting the card in the scene.
I avoid Auto White Balance because it changes the WB shot-to-shot based the assumption the brightest tones under clipping are neutral. It will work reasonably well if there is a big white shirt or wall in the scene, but if the shirt or wall are actually light blue or yellow AWB will shift the color in the opposite direction (on the color wheel).
Mixing flash and ambient creates a different set of problems. The flash lit foreground will have different color balance than the background, and if there is overhead spot lights the reflections on the tops of heads and shoulders. The solutions in mixed lighting situations are:
1) Overpower the ambient: Flash exposure is not affected by shutter speed, so increasing and decreasing shutter time will make the ambient light contribute more or less light. A speed of 1/60th might show the background detail in a yellow / orange tone in tungsten light. Changing speed to 1/250th will darken the background and the yellow bias will not be noticed as much.
2) Gell the flash: Gelling the flash to match the ambient, then setting custom WB to the gelled flash or using the ambient light WB preset will allow the two to blend seamlessly. This is a good strategy when a brightly lit, normal looking background is desired.
3) Blended layers: If shooting RAW it is possible to take one RAW file, make a copy for and balance each for every type of light in the scene, then blend them with layers and masks over the copy balanced normally for the main center of interest in the photo which is often what has been illuminated with the flash. This is the most time consuming, but with some basic Photoshop chops can produce shots with seamless color similar to the scene by eye in person. Keep in mind in person the eye will adapt as it moves around the room, but will still perceive differences in table lamps, windows, etc. If everything is balanced too close to neutral it tends to look fake.
Neutral WB is only a consistent starting point. Color and color contrast are one of the things which trigger emotional reactions. Depending on the content and context it may be desirable to go cooler for a winter scene or warmer for a portrait, or cooler and warmer in same photo, such as photo of someone sitting inside by the fireplace looking at the snow falling outside the window. If this is a new concept for you, pay attention next time you watch a movie because cinematographer and production designers are masters at the art of conveying mood with color balance of the lighting and set design.
There are situations like sunrises, sunsets, warm afternoon light, etc. when Custom WB should not be used. If the goal is to capture the ambience seen by eye what I do is use the Daylight preset to create a consistent baseline then adjust by eye and batch correct when editing the files. For close-in portrait work in those situations the most "natural" result is achieved by using flash to light the faces making them more normal perceptually, with the camera WB set to flash which will render the background a bit warmer than seen by eye. Shoot at least one test shot with a gray card held next to the flash lit face as a reference when adjusting the color when editing. The most important thing is getting the faces looking more or less neutral, but in context with the natural lighting.
Thus there is no one single "magic bullet". Just start with the goal of the emotional reaction / mood you want the lighting color to evoke then work the problem to find the most effective strategy to achieve it.
The best cure is an ounce of prevention. Avoid shooting under trees and around foliage or brightly colored building which will bias the color the the daylight. Your eyes will adapt to it and it will be difficult to notice, even when the shots are viewed on screen. That's especially true of the green bias under trees. Green light + pink skin = gray dull, flat looking skin that looks to the untrained eye to be under exposure. But correct the WB to neutral and the contrast improves along with the color. That's exactly the type of situation where a gray card next to the face in a test shot is an invaluable reference.
