dundonald wrote:
I'm laying flooring [1] in my loft. The wiring is simply thrown over the
joists rather than a hole drilled centrally through the joist and the wiring
passed through the hole. 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 tried searching the archive of this group for the answer, but after a
couple attempts trying to guess what others might call what I call a 'nick'