Ideally neither. The FMS should handle cold weather compensation, including flight path corrections as necessary. In fact here in Canada it's required that all new or updated FMSs have cold weather compensation built-in.
As such there is an accepted ICAO-blessed algorithm used by FMSs to compute these corrections, published as part of the PBN RNP standards. Unfortunately I don't have access to these standards to read what they say, but may be you can get them from your company. The algorithm might be a simple linear approximation, but maybe not.