Have to admit I've been doing this for as long as I can recall (since software mod). I know (as AerocatS2A has already advised) it apparently gives improved climb perf. predictions albeit I can find no FCOM reference other than on FCOM-DSC-22_20-50-10-28-INIT A PAGE that has an entry stating that when the "FROM/TO" fields are utilised (or CO RTE), then the defaulted value is the airport of origin ISA value.