Based on the thread titled "Camera Calibration?", I thought the following might be interesting to some. While I was using the 20D, the same information applies in general to all of the digital SLR cameras.
When you are shooting under more or less fixed lighting situations, then it is often wise to use a custom white balance rather than just AWB. For the 20D, you take a photo of a white or gray card and then make that the reference photo for the 20D Custom White Balance setting.
Contrary to what many will say, it makes no real difference whether you use a white card or a gray card for the reference image. Here are two photos which illustrate this point:
Both of theses shots were made in identical CFL light. These are the actual images, cropped and resized, but otherwise as is. I changed the custom white balance in the 20D between each shot.
Just for reference, here is the same shot with the same CFL lights shot with AWB:
If you shoot RAW only, then you don't need to worry about White Balance. But if you shoot combined, then it is wise to get the white balance right. That being said, I use Custom White Balance whether I shoot RAW or not.
Another note: Place a white coffee filter over the lens and shoot the scene in the light you're balancing for. Use that image as the CWB reference image. Gives the same results as above.
Also note, that it can be very helpful to calibrate the meter on the camera. I use a Black/White/Gray target for this and the camera's histogram. I find the 20D much closer to the proper exposure than the 10D was. But my flash meter is generally up to 1 to 1 1/2 stop different, so I adjust the camera appropriately.