The FMC is just another instrument to help us manage the flight - entering this altitude or another for A/I on or off is irrelevant.
It´s not really irrelevant and I think you´l find Sudden Winds is probably correct, that it saves fuel (although probably not much). It wouldn´t save fuel if you were in a mode other than VNAV, but lets face it, most aircraft leave their cruise altitude in VNAV.