Roger has posted a very objective, well thought out article on lens quality and manufacturing tolerances. It's definitely worth a read if you have a few minutes to spare.
"After we published a number of posts about copy-to-copy variation, people were quick to say that this company or that needs to 'just' improve quality control. I totally agree but realize most people don’t have a clue what ‘improving quality control’ would really look like. I think they have some vague idea of hiring a guy named Joe to sit at the end of the assembly line, check all the lenses, and reject all the bad ones. Optical quality control for lenses is way, way more complicated than that."Check out the LensRentals Blog for more info.