The two most hysterical bans I'm thinking of, the one in California and the one that they're trying to but likely won't (hopefully) get through in Maryland this year which mimics it, gave current owners a grace period to register and keep what they had. (In California this has resulted in an estimated 300,000 law-abiding citizens becoming de facto criminals because they didn't register what they had.) Other than that, probably wouldn't hurt to save your receipt or packing slip. A postmark isn't going to prove what was in the package and a newspaper photo only proves the picture was taken after, not before, a certain date. I like the idea of stamping the date in the receiver as well, even though it can't be proven its a true date it still looks good.
Right now public opinion seems to still be swaying in favor of gun owner's rights and away from the likes of the Rebecca Peters (aptly named) and Bim Jrady's of the world. But I also think they sense an opportunity to push their agenda with the growing anti-conservative backlash being whipped up by the left.