Adjusting the time (speed) vs fuel burn for a minumum cost is not pilot`s job if CI is provided by company.
Fly the CI speed at any phase of flight for minumum cost. As entering CI to FMGC, we are telling the computer the comparison between Fuel Cost and Othet costs (time related Costs). So, computer optimizes the best speed for minumum cost with real-time effects (wind speed, temp, altitude, weight, remaining route length etc.)
Full Stop