If the snow is heavy and can support the weight of your Sammy, keep the tires inflated more (perhaps 10psi) so you have extra ground clearance. However, if the snow is soft and deep, and you're going to be plowing through the snow anyway, so air down the tires to as low as they can go and still stay on the rim, and you should find that you have gobs more traction.
The snow conditions are what determine what you need to do with your tires. In the snow in the accompanying pictures, I wasn't making any progress at 7psi. The tires just spun and dug. Since I was going uphill, I needed as much forward traction as possible, and as large a tire footprint as I could get so I'd float on top of the snow. Speed also helps keep your tires up on top of the snow, rather than sinking down into it and burying the axles and frame.
Aired down to half a pound, I had enough floatation to stay as high up on the snow as possible, and the tires flexed enough to easily clear the tread of the snow, so I was able to plow my way through. And remember, those are 33x14.50's with about 8 inches of lift buried somewhere under all that snow, yet I was coming at my buddy with the camera so fast, I about ran him over. Experiment with your air pressure until you find what works best for that day's snow conditions. And if the sun comes out, the temperature rises or falls, or if more snow accumulates, the snow conditions can change quite rapidly. Carry an air compressor with you.
-- Geoff Beasley