Mark,
I'm curious as to what "absolute maximum accuracy" would consist of? As I understand the WB dropper, all it does is correct the color balance so that the location you've clicked on is neutral gray. Doesn't matter whether you click on a spot that's light gray or dark gray, it just makes it neutral (ie. R=G=B). If you want any kind of accuracy (whatever that is), I would think you would have to have something in the scene that is known to be neutral gray. I use a WhiBal for that purpose as most things that appear to be gray or white are in fact not really neutral. But then, I often modify the white balance with the sliders after I use the WB dropper for 'artistic reasons'.
To get back to the OP, for snow scenes, I find it difficult to find a portion of the snow that gives me a realistic balance. In particular, be wary of using the dropper on sunlit snow that's already nearly white; as mentioned previously, if one of the channels is blown out, the WB dropper won't work correctly. Also, be careful of using the dropper on shadowed snow. Snow shadows are generally quite blue under a blue sky; making them neutral gray will make the scene too warm. My opinions only, of course. In the end do what makes the image look the way you want it to look and don't worry about accuracy. Cheers,