The engine manufacturers agree that the worst case TIT occurs during climb around the high 20,000's. The early B747 with P&W JT9's regularly demonstrated this. Hence derated climb thrust can and does reduce turbine distress and promote hot section longevity.
That may be true, but is not supported by the automatic operation of the CF6. Any Climb thrust derate is gradually reduced to 20,000'. Above 20K full Climb thrust is used.