Gene, what are the advantages and disadvantages of stepping through the ceiling? If you're never going in the attic, not going to store anything in the attic, then there's no reason to install plywood. If you have 6 inch ceiling joist and have 12 inches of insulation, then the plywood would compress the insulation, eliminating it's insulating value. Twelve inches of insulation compressed to 6 inches is of less insulating value that 3 1/2 inches uncompressed. Attics weren't designed to store things in, but we always do and create our own set of problems. Actually, if you have a fairly large attic, it would be cheaper to build a storage shed than floor it with plywood.