On boeings derated takeoff is most of the time combined with reduced climb thrust. Of course cruise thrust and fuel flow is the same, however with reduced take off and climb thrust the airplane takes more time reaching an efficient cruise altitude and therefore uses more fuel.
However the difference is not much, at least on 737s. On our route network which ranges from 40 minute to 6,5 hour sectors it is only an average 40 kg per flight which can be saved if pilots select full climb thrust instead of reduced climb, another 10 to 15 kgs could be saved if using full take off thrust, however the reduction in maintenance cost is a much bigger factor there. Those 40kg per flight are well above a million euro per year though.