general rule?

A neutral is not allowed to be switched. (I think I remember an exception regarding fuel pumping.) A white wire is not always a neutral. It sounds like you are referring to what the ignorant among us call "back-feeding the neutral". The code does not allow the use of a white wire as a "hot" at the point of connection to a load, so if we're using something like NM-B to make a path from our supply to a switch and back, we have to use the white wire, (not a neutral unless it is eventually connected to the neutral buss), to feed the switch, and the black wire to feed power back to the load.