the "break off a piece" lens question

[serpaco007]

I just bought two old lenses for my 7d.

nikkor-n 24mm f2.8
kiron 105mm

I've seen a few posts where some people say you need to break off a metal piece or remove a stopdown lever, but they were always deep in a thread, and nothing was clear. The implication is that it can damage the mirror, i believe?

I'd like to dedicate a thread just to that and weigh in everyone's comments.

Thanks guys!