Sorry Beazlebub its not that simple,
You clearly have not flown a 757 or 767 with boeing optional company fixed derates installed. Thomson guys among others know all about it. If you try and preselect climb 1 on the ground you actually select the fixed derate and then subsequent assumed temp reduction gives you what we call a double de-rate. (ie you derate the fixed derate) The trick is in the eicas. Selecting 1 or 2 on your machine gives a WHITE 1 or 2. On fixed rate machines the fixed rate shows as an all green TO-1/2 etc when 1 or 2 are pressed. Boeing have a huge number of optional extra's available. Your aircraft are just one of several different fits
The 777 EICAS thrust headings are exactly the same using fixed derates then assumed temp. (climb thrust selection is much cleverer though). All selections through the FMS of course.
Why wouldnt you want to use as little thrust as possible and still meet all the performance criteria both from a commercial and risk reduction point of view