jungle drums,
At any time, the crew can always use higher power settings as they see fit.
This is normally only done when there is a reason to do so such as when adverse weather conditions exist at or near the airport etc. To use additional thrust under normal conditions is completely unnecessary due to the margins already provided. In any case, full rated thrust (and more in fact) is always readily and instantly available should the crew need it during takeoff.
To use max takeoff power always will result in significantly reduced engine life and increased risk of failure. It is akin to "flooring" the accelerator at the traffic lights in your BMW every time they turn green!