Low PSI absolutely can help in the snow. I was stuck years ago in a fwd 2wd town & country similar to the one we have now, but without track control. I used to commute 450 miles per week with regular winter snows, but ran A/S hwy tires year around.
In that car, with narrow, 70-series tires, it had to be dropped to about 12 pounds. So, it limited your top speed in places where traction may have been better, and it was hard on the tires.
BUT, it worked and worked well as a last resort. someone mentioned above that this could be detrimental-- snow tires cut through. I was on hard-pack snow with ice beneath that... there was no cutting through, just spinning. Dropped all that pressure and it allowed me to crawl home. I think we ended up with ~3.5 feet that night.
Not sure if this is best in all situations, like mentioned above. And you'd hate to be wrong and then be stuck with de-aired tires wanting a pump....