Aside from the derating mentioned above--- the use of reduced thrust procedures allows for a constant thrust weight ratio minimizing engine wear--but worse in some cases for noise abatement---and perhaps a very negligible effect on fuel savings---although too complex to say which way
--- less thrust less fuel--but more time to clean up--more fuel, hence negligible effect--more for maintenance---but many takeoff will be too close to maximum TO weight for use of that procedure anyways
in cruise as also mentioned before complex cost indexing schedules are required