It seems to me that inherent qualities like tagging, watermarks, etc. that cannot be removed from stamps make for legitimate varieties in the same way that perforation, overprints, and color differences do. You can see them, so the stamp is different. It's the back of the stamp I'm talking about. That backs of stamps are an interest of so many collectors has always surprised me. I don't mean damage. Thins or pinholes or other damage affects the entire stamp, not just the back. Even gum missing from the back of a stamp (so not "mint") is of little interest to me -- at least for 19th century stamps. And most collectors agree since stamps without gum continue to sell very well. Consider this: If the gum on the back was originally clear, but later it was slightly less clear with just a tiny bit of yellow in it, would you consider that a collectible variety? Perhaps, but then shouldn't you at least mount those stamps backwards to show the difference? That does seem a little silly, I'm sure you'd agree.
Related to this is the whole MNH obsession that exists today. This got ginned up by certain dealers in the period when stamp prices were rising quickly c. 1970s as yet another way to sell even more expensive stamps. To me, it's up there with "slabbing," entombing a stamp forever in plastic to keep it "perfect". That originated with coins and baseball cards. These were basically investment-related schemes by some dealers that had little to do with collecting stamps. Does anyone actually collect "slabbed" stamps?. The idea that a MNH stamp is "purer," more untouched, than a mere MH stamp allows it to be sold for more money, often double or triple the price of the MH stamp. That seems absurd. That so many collectors jumped on this bandwagon, as I said, has always surprised me. But live and let live -- if collectors like spending more money for perfect backs of stamps, then go right ahead -- plus I get to buy all those "mint hinged" stamps no one wants. Unless the MNH fixation goes away someday, it's true those stamps will also sell for higher prices, but as a reason to buy them in the first place that is tautological. "Pay more so you can sell for more" is not a very good investment approach.
But back to the backs of stamps: Besides the back of stamps not being visible, once the backing paper is removed, is that stamp still a variety? I don't think it is. So it depends on the backing paper. If you collect with the backing paper still on the stamp, do you then display the stamps backwards to show their differences? Belgian stamps often had tabs on them ("Don't Deliver on Sunday," advertisements, and so on, so collecting those stamps -- with tabs intact -- makes for a variety just like perfins, color varieties, and other differences do . But you can see all those things. You can't see the backing paper. You could always remove the different types of backing paper from your stamps and display them in mounts on your album pages. I'm kidding. No one would seriously do this any more than they'd mount their backing paper stamps backwards. This is what makes me think these "varieties" don't really matter.