Isn't the idea of the assumed temp method that it reduces wear on the engine such that x number of years down the line someone doesn't set rated thrust then have it go bang at an inopportune moment hence providing a safety margin in itself although it appears somewhat immeasurable. As has been mentioned one always has the discretion of setting full thrust at any stage without any penalty. What are ssg's thoughts on taking off at RTOM which is field length limited given the assumptions that go into how heavy the aircraft is declared to be? This would have the same effect on an assumed temperature take off if I understand it correctly but at least you could be happy in the knowledge you have full thrust set.