The procedure on Jepp CA-19 is quite clear. In a nutshell, the baro altimeter reads high when the temps at the reporting station are low. It IS necessary to make adjustments for this.
If flying an ATC assigned altitude, ATC will take this into account, so if you "hear" an altitude, fly it as indicated.
If you are on pilot nav, and "read" an altitude (off a Jepp chart or other printed source), then you must apply the correction (and inform ATC you are doing so). This correction may be minimal (as in an ILS DH) or many hundreds of feet (as in intermediate segments on published approaches).
Checking some MSAs in high terrain conditions will make the Cold Wx corrections critical. Your true altitude may be very close to the rocks. Even is there is no high terrain around, there are some very tall TV towers, and your radio altimeter won't help you miss those at all.
RA is a good awareness tool, but not to be relied on for these purposes.