Well, it's not going to be terribly accurate, but it will get you in the ball-park. Lacking anything better, I've been known to use either one ('though the eyes are more likely to be a truer white than some people's teeth).
You don't need white. Neutral gray works too, which is a large part of what gray cards are all about.
The key is being sure all 3 color channels have the same value.
White is 255, 255, 255. Neutral gray is any shade of gray that also has equal RGB color channels. 200, 200, 200 is neutral gray, as is 176, 176, 176.
209, 209, 209 is 18% gray.
Unlike the ACR that is Camera Raw, the ACR that is Lightroom's Develop module doesn't show the RGB color channel values at the lower left corner of the histogram.