Not a self appointed expert but I would assume if your airline was that worried about it, each flight segment will have an assigned cost index to follow, playing with selected speed and V/S is changing the CI, having said that, pilots seem to enjoy manipulating the climb and descent to suit their needs, for instance, our base is close to an FIR boundary, we try to help the controllers by reaching our cruise level prior to that boundary, relieving the need for coordination with the next region. Another way to answer your question is by simply inputing your CFP data for that length of flight, now, whatever CI you used, now change it to "0", now compare the burn on the INIT page for your answer. Remember, operating cost is much more complicated than saving fuel.