I have come across NEMA Bulletin No. 92 which states that when installing a recess light fixture with a required supply conductor insulation rating of 90 deg. in a house that is wired with type NM cable ( 60 deg.) you should install a junction box 18" away from the fixture and switch over to type NM-B cable ( 90 deg.) to run into the fixture. Is this a common practice? Does anyone really do this?