When you use two lighting sources...such as natural light with flash fill...or I know Dew uses a softbox and slave flash....which white balance to you use?
The auto white balance of the 10D is usually pretty accurate - Tungsten light can give you some trouble though. Refer to p. 52 of your manual "Custom White Balance" - this will give you perfect results...
If I have critical stuff to shoot, I photograph in RAW mode and correct the WB in the RAW converter if necessary (I use Photoshop CS)