I'm not an electrician, but I turn of the Main. the main (no pun intended)
reason is that there are a lot of electrically hot surfaces in the box, and
if you're fishing in romex wire, particlarly with the bare ground, it seems
too easy to me to bump something and get a nasty shock.
If I were doing this for a living, on a daily basis, I'd figure out how to
do it with the juice on.