I would suggest airing UP, not down. Unless you're in deep snow with no chance of getting down through it, you want HIGH contact pressure to open up the tread and bite down through the snow/slush/mud to get traction, otherwise you're more apt to spin/slide on top of the stuff you compress under the tire and get nowhere. If it's a heavy wet snow, air up. If it's a deep fluffy powder snow, air down.

Brent