Steve Wilson wrote:
Is it good practice to cut a little nick out of
the joist, enough to lay the wiring for the flooring to flush with the joist
over the top? Is this detrimental to the structure?

Yes.
Cutting out a 10% section of beam at the top reduces strength by at best
20%.
What about routing a groove in the chipboard?

I've cut out the odd small notch in my joists for this purpose, but
always felt that screwing T&G chipboard across the joists would more
than make up for any weakness created by the notch. Am I dangerously
wrong?

Probably wrong, probably not very dangerously.

For the tounge and groove to add to the strength, it'd need to be bonded
to the wood beneath.
For roof joists, the top is in compression, and the bottom is in
tension.
If you cut out a notch at the top, that relaxes the beam at the top, and
means that for a bit the very top of the beam does not contribute
to the structure.

You can only make this up with the T&G if you jack the beam up to its
original condition (so the gap where the notch is cut out is spread back
out to its original size) sand the beam top flat, clamp and glue the T&G on.
Simply screwing/nailing won't do much, or gluing the T&G without jacking.

If you fix it back, with the beam notched but without stretching, then the
additional material is not stressed, so it's not contributing anything
to the structure.

Plus, the T&G will be weaker as the grain will be going the wrong way.