Dan,
"It also may help to shoot the smallest groups with a non-AO scope."
Which is exactly why I have sometimes used it. The last was a few months ago, when assigned by a magazine to test a new bolt-action .22 rimfire hunting rifle for an review article. Despite owning several dozen scopes, a lot of them were already on rifles--including two of the several "test scopes" I keep for such use, which were adjustable for parallax. (As a general rule, there are around 3-4 such test rifles in the safe at the same time.) So I mounted a Swarovski Z3 3-9x36, and shot the .22 extensively at 50 yards with several kinds of ammo, from hunting hollow-points to match stuff, backing off my eye so the reticle could be centered. As it turned out, the rifle would shoot 5-shot groups of half an inch or less with just about any of the ammo, so I decided to buy it from the manufacturer--and then purchased a parallax-adjustable scope to put on the rifle, returning the Swarovski to its usual job of testing centerfires.
On another occasion I was sent a custom centerfire rifle for review, which came from the maker as a package deal with a fixed 10x that turned out to have apparently been set at the factory to parallax-free at 200 yards. It would have been easy to do the testing at 200, but the magazine that assigned the article uses a specific testing protocol that, among other things, stipulates 100-yard group shooting. So I backed off my aiming eye when shooting the test targets, and the rifle shot fine.
