Saw a thread a little while ago called something like "Flex or Full power which uses less fuel?"
Seem to remember that the result was.
FLEX takeoff uses lower power, but uses it for longer to get to target altitude, total result is marginal INCREASE in fuel burn, but a decrease in maintenance bills, as engine core doen't get as hot.
Good search term to use on this site:-
FLEX (Airbus speak)
Derate
Assumed Temp(erature)
Also "Cost Index" which is a whole new topic in itself, basically your airline decides how oimportant 2 factors are to them, Fuel Price, and Hours in the air (Maintenance Price) you then dial a number between 1-100 (or is it 0-100) into the aircraft and it picks a speed to cruise at accordingly. Fuel price high - go slower, Fuel price dirt cheap - go faster.