FMC selected De-rate is called T/O de-rate - this applies ONLY for the takeoff phase of the flight ie up to 1500' at which point CLB thrust takes over - assuming all engines operating etc. (Having read other responses about the change over at lift off - I agree, however the actual thrust setting doesn't change until 1500 when N1 annunciates...if you get my drift!)
At 1500 ft when N1 annunciates on the FMA the takeoff de-rate is effectively removed and the aircraft sets the climb thrust selected - if you are using a de-rate you would normally get CLB-1 or CLB-2 so that you don't get a thrust increase when it goes to N1 mode.
Regardless of the thrust rating indicated on the FMC ident page if you "firewall' thr throttles you will get 24k for a 700 and 27k for an -800 (VOL 2 7.20.4)
T/O de-rate is considered a limitation for that TAKEOFF - ie you shouldn't push the levers forward because of the reasons stated by John Tullamarine.