For all the discussion about which camera white balance setting to use, I think it is important to note that, if you are shooting RAW, the simple answer should usually be "Always use AWB". The reasoning for this is because white balance is an easily correctable thing in post processing when you shoot in RAW. Even if you do use the "Manual" or "Custom" white balance setting of the camera, the camera is still making an educated guess...it is just making a more informed guess. Custom WB can still result in incorrect color balance, and there are times when it can be as off as AWB.

In contrast, using AWB in camera, and correcting white balance during post processing with RAW, gives you FAR more control over the final white balance than you can possibly get by working with in-camera tools. When using a tool like Lightroom or ACR+Photoshop, you have the option of using a "white balance color picker tool" to select the area of your image that actually is white, and the software will correct the rest of the image from there. It is difficult to get more accurate than that.

Additionally, if you take a "white balance baseline" photo wherein you include a gray card in your scene, then remove the gray card and take the rest of the photos for that particular lighting scenario, you have even more accurate control over white balance during post processing. Simply use a white balance picker tool, select the gray card. Copy the white balance setting from that initial baseline photo (after its been corrected) to the rest of the photos shot under that lighting to apply the correct color balance (in bulk, if you have a tool like Lightroom.)

The case where AWB won't necessarily work all the time is when you can't or don't shoot RAW, and use JPEG instead. Correcting white balance in a JPEG during post processing is difficult at best, and can be impossible at worst. In such cases, you might try to use a custom/manual WB setting. If your camera supports it, you may also want to tweak the custom WB offsets (color temp. along the blue/yellow axis and color tint along the green/magenta axis) to improve the results.