Things changed a bit with the new FMC update. If the perf init page on the ground is asking for the actual temp at the cruise leveg eg. -56 iso isa deviation, then following will happen:
On the ground:
FMC does not know actual conditions at the cruise FL, so its calculating with the given (anticipated) figures (weight, temp, cg)
In the air:
FMC calculates with actual temp and weight and the entered CG, this way taking care of changes of temp along the route
If you want to be even more precise you can calculate your actual cg...
(Only for new FO´s in line training..)