If you are fishing for an afternoon/evening you could get away without blocking, but if it will sit on the ice for longer definitely block!
Like Jonny said, 2×4’s will suffice, but as the season goes on my shack will have several blocks under it. My shack stays from December thru the end of February in one spot.
Last year was the worst, heavy snows and major weight on the ice, especially from the snow banks all around the house. I showed up to the house expecting a water problem, drilled a hole and I had to block the house up another 12″ with whatever I could find…2×4, 4×4, even chunks of 6×8’s. Water flooded the entire area and I spent the next couple hours running back and forth getting supplies. Never did fish that night. I had to let it freeze overnight, returned the next day and re-banked the house.
Then at very late ice I return to chisel out and retrieve all the wood. My wife thinks I’m crazy, but it’s worth it regardless of all the work involved.