Well, it washes out to full climb thrust of the full engine rating, in case of a 18k derated 700 to 22k climb thrust. We do not use MCT anywhere except in an OEI case although climb thrust and MCT are pretty similar. The fixed derates are only used for take off until thrust reduction, not during flight. Therefore the climb thrust settings are based on full climb thrust, not on those fixed derates.
Unrestricted climbs always save fuel compared to step climbs, however unrestricted climbs at full climb thrust from thrust reduction on (in our case 1000ft) saves even more fuel as the efficient cruising altitude is reached earlier. Besides it provides a smaller noise footprint as well.