Econ speeds for climb cruise and descent vary according to cost index, which (if it has been calculated correctly!) is hourly aircraft cost divided by fuel cost. Thus, a low cost index indicates high fuel costs and the FMC gives a minimum consumption profile. High cost index gives max speed.
As fuel costs vary from port to port, the most efficient procedure would be to calculate a cost index for each leg. In practice, most operators come up with an averaged number, which they review from time to time.