320-
I agree with bizjet. In a nutshell, if you "read" the minimum altitude (as from an approach chart or a map), you should/must be aware of the error and apply a correction. If you "hear" an altitude (as assigned by ATC) then you should fly it as assigned. Different countries have different methods of correcting assigned altitudes for cold weather, so if in doubt, ask.
On the RNAV approaches, the ones with which I am familiar say "Not Authorized below -15C." In other words, I think you have it backwards. The corrections should be made on those approaches until it drops below -15C, at which point it's NA.