Using an assumed temp does cost you more fuel for that flight versus using full thrust and getting to altitude quicker however the engine wear is significantly reduced saving on maintenance.
Over the life of the engine running it with max reduced thrust possible saves fuel as SFC is directly related to EGT which in turn is related to engine wear. The greater the engine wear and stress (high thrust settings) the higher the EGT rises over time and therefore the more fuel the engine will use all the time.
Therefore using reduced thrust where possible does in fact save fuel in the long run.
This is all contained in a CFM guidance document.