I am sure on my little 737-300, we derate...assumed temp etc. for all the same reasons as everyone else. But then my experience differs slightly to other peoples( I think).
On passing 1500' we select climb thrust which then annunciates R-CL on the magic boxes and hey presto we get a REDUCTION in thrust(3% I think). I am sure this to save a few more pennies and for noise abatement requirements. Then at 150 the thrust automatically changes to CL to give you the extra welly at higher levels.
Other responses have been implying that they get an increase in thrust at climb thrust selection..please correct me if I am wrong but I thought we went with a bit more on the ground to get us airbourne and then reduced at a safe height as the extra ooomph(my technical term for that particular stage of flight) was not required.