VNAV
You will generally be safer if you do not modify the altitudes in the FMC. You will generally get the reason as " chances of corrupting " the Fmc. Although the likely scenario would be the chance of error insertion while modifying the altitude constraints.
The best way to fly a VNAV is to fly an approach procedure that is approved to fly under cold weather conditions instead of standard ISA temp. As more and more carriers are discovering VNAV you will see more approach charts offering temp limits for using VNAV e.g valid uptoISA -15 0 or ISA -30 etc.
Ideally that would be the way to go .Ie dedicated VNAV approach procedures ,however we still need to cater for the larger GA aicraft operators who still employ the duck and drive approach for Non Precison approaches. If only a cheap solution for VNAV becomes available will we see a global push for stabilized constant descent approaches, and would ultimately be reflected on approach charts
BM