Changes to Altitudes.
Ray, you are permitted to alter the heights in the FMC but only in the case of temp. correction. No other alterations are allowed.
Applying a correction at -25C, not something most of us experience very often, makes a difference of 490' to an MSA of 3000'
While you probably won't hit anything if you are 500' lower than you want to be, you most certainly will be well below your MSA and obstacle clearance will be compromised.
I will continue to apply the corrections, as recommended, and if ATC ask me what my altitude is I will tell them the figure with the correction applied.