I'm not a huge fan of it anywhere public. But should someone not have a lounge bathroom access (or available), I would prefer a big sink with plenty of space and lots of space, and toilets on the ground than anywhere on a plane. FWIW, my only shave while travelling was in a UA club shower after about 21h travelling; in J.

The rule is simple, for shaving, nail clippings etc...
If you wouldn't do it at home, leave it out of the airport (lounge washrooms are fair game but if you make a mess leave a decent tip!) are and definitely out of the aircraft!