When correcting DA or MDA for cold temperature I understand it is on the PFD, not on the FMC, that it is not allowed according to the manuals.
From FAF/FAP onwards we are not allowed to modify anything, or else we are messing with the vertical path, that is clearly stated on the manuals (B737NG).
the path calculated by the FMC should, as pointed out by some, take in account the worse case within the limits of temperature on the chart, but the Minima at which you initiate your Go around needs to be increased on your PFD.
Do you correct DA for an ILS if it is -20C?, I take that the answer is a yes, same should be done for other type of approaches.