Different outfits, different rules...
In some cold weather correction is to be applied below 0C.
In some aircraft types it can be selected in FMC.
In all cases where it cannot it need only be applied to all MSA and below altitudes, thus anything in FMC which depicted on a chart is at or below MSA requires a cold weather correction.
For RNP approaches, different rules apply inside the FAF, where no corrections are allowed, to make it more awkward.