There's a zillion considerations for choice of thrust rating as related to aircraft performance, primarily related to Vmcg, wet, contaminated runways etc., but to consider the commercial implications.....
If your engine has 3 thrust ratings, i.e. 22K, 24K or 26K, and reduced thrust (Assumed / Flex) is available for all of them, consider a day when 20K is required for the particular weight and performance scenario.
Any of the 3 ratings with an assumed temperature will satisfy the 20K requirement, e.g. 22K @ 40°, 24K @ 45°, or 26K @ 50° (I just made those numbers up) will all produce 20K thrust at the same EPR / N1 / N2 / EGT, thus there is no difference in engine wear regardless of which rating that you use with the appropriate Assumed Temperature.
Manufacturer's guarantees do not consider thrust reduction, the benefit gained here is in the hangar with reduced ACTUAL engine wear. The manufacturer ASSUMES that you do use the full value of 22K, 24K, or 26K, and does apply the guarantees applicable to the number of occasions that each rating is used.
There are therefore 2 savings -
(1) Using Thrust reduction (Assumed / Flex) reduces actual engine wear, and reduces maintainance costs, AND
(2) Using the lowest RATING possible (22, 24, or 26K, regardless of thrust reduction) increases the warranty from the manufacturer, further reducing costs with an improved guarantee.
If safety of operation calls for it, to hell with the cost, use the highest possible thrust consistent with safety
but plan it in advance and work the V speeds appropriately. If you're doing a 22K Takeoff on a wet runway with lower V1 speeds, and go to the full 26K at V1 with an engine failure, you're gonna lose it!
Except in the rare cases (al low weights) where V2 is governed by Vmca, there should be no problem in advancing to full thrust following engine failure
after V2 has been reached.
Regards,
Old Smokey