The answer is yes...technically.
I personally don't like doing it, and if a job can wait, I like to put the job off until the snow is gone around the stump, and the ground has re-frozen.
With that being said, yes I can grind stumps with snow on the ground. My machine is a 4x4, so a little slippery snow isn't a problem unless the stump is on a hill. The snow does make for a messier cleanup however. Usually I blow of the entire area of grass to remove as many small chips debris as I can, but with snow on the ground, I tend to leave more of that behind as the snow hinders my clean up.
So, in closing if you do have a stump on the ground and want it gone ASAP, call or text and we can schedule something.