Something of a philosophy question here...

I've recently moved into a 100+ year-old house and, in addition to
finding the usual decaying wiring (mostly BX), I have discovered all
sorts of poorly done and potentially dangerous work (mis-wired
receptacles, neutral-switched switches, grounded receptacles without
grounds, etc, etc). I believe I can handle most of the fixes for these
things, but I'd like to comply with the laws and get permits and have
my work inspected.

The question is, in the opinion of the "old hands", where is the
dividing line between work needing a full permit and a simple "repair"?
Replacing a receptacle: sure, no need to pull a permit, but what if I
replace a corroded 6-foot BX run with some fresh NM at the same time?
I think there is a gradation there, but I don't have a feel for its
parameters.

Also, I have some lingering uneasiness related to 3rd-hand stories of
inspectors demanding whole houses be brought up to code, whenever any
work is done which disturbs the "pre-existing" state of the wiring.
While this sounds counter-productive (since this would disuade people
from doing any work, when even a little could really enhance the safety
of the wiring), it doesn't mean it doesn't happen. Or, maybe this is
mostly paranoia on my part?


Anyhow... any thoughts appreciated.