I think this is an area where the code does not specifically say something .... kind of gets back to the "this is not an instruction manual, and assumes you already know what you're doing" part of the introduction.
I believe that the NEC allows NM to be run in pipe for protection against physical damage. I believe that the NEC further requires that NM run exposed below a certain height (7 ft?) be protected.
Combine the two, and you wind up with plenty of panels and furnace disconnects having stubs of pipe coming out the top.
If I remember correctly, I don't think the NEC requires any fitting to soften the edge of the pipe ... though that would clearly be a good idea.
This may be a carry-over from K&T days: