A few years ago I did a piece on custom white balancing your camera. It is important to learn this skill to get the best looking footage, especially if shooting with an 8-bit 4:2:0 camera (like many of us use).
Interestingly, as I was doing a little research to put this piece together, I noticed that using a gray card generally produced better results than using a white card. I've had a lot of questions on why and up to now, I just assumed that it had something to do with the fact that 18% gray is closer to the luminance level that we generally shoot for skin tones. Turns out that was a decent guess, but not the whole story.
Art Adams posted an interesting piece titled, "The Secret Art of White Balancing" that goes into way more detail, particularly in light of the fact that many cameras can now capture 12-14 stops of dynamic range. To do that, cameras have a tendency to compress the highlights and shadows (even aside from log profiles, it seems) so using a target closer to the less compressed mid-tones often results in better white balance.
Definitely worth the read, if you're nerdy like that (I am).