I think you can mess around with lenses (or films or developers or darkroom techniques) as well as take photographs, it's not an either/or unless you want it to be.
As for shimming, the issue is that Soviet LTM lenses were not exactly the same registration as Leica, plus they were known to be a bit variable in accuracy to their own standards. So mounting a Soviet LTM on a Leica (or Voigtlander) body might mean you ended up slightly off to infinity focus, or maybe a bit of a front or back focus problem dependent on aperture.
You can adjust this by adding or removing shims within the lens.
I think when Brian dismantles any lens, he'll check for such things and would correct it as a matter of course. Many of us I'm sure probably wouldn't even notice the difference between most lenses whether shimmed or not, unless the problem was really quite gross, but Brian likes things just right.
I'm sure there's a zillion threads on shimming if you care to google