Manual White Balance “K” can be very useful when you’re shooting with fixed lighting, and don’t want to have to edit every single photo just to make the WB right. Here’s a video that explains how to manually set your WB with a Canon 7D.
https://www.youtube.com/watch?v=ZhXVSoOoDW4
Obviously he will want you to buy his product to use, but you can be a little creative and find something that will work for getting a sample shot. 🙂 I have an ExpoDisc that I picked up for a song, and never actually used yet. :~)