If there are no airspace / ATC issues, the optimum descent is achieved with thrust lever closed. As I said earlier, the AP will be used to maintain a Mach number or IAS and the ROD will do its own thing as I described.
An FMC can be used to calucate all that lot, taking into account airspace constraitns and forecast winds, and the Top Of Descent commenced accordingly. Various types of managment can be used to account for variation in real life from the plan.
e.g. if the winds are not as forceast, something has to give. Either the aircraft will not descend on the planned profile (but can maintain the specified speed) or the speed is maintained (in which case the profile is lost).
pb