santiago15,
Correct within limits. The Flight Level for maximum speed is at the CAS/Mach changeover height. Any flight higher OR lower will result in lower speed. Thus, speed is often-times better below Optimum Altitude, but not always. Fuel burn, however, will always be worse above or below optimum altitude, with the greatest penalty (for the same vertical displacement) above Optimum Altitude.
CV Donator,
The minimum fuel burn will be achieved if the aircraft is kept at it's MRC speed (or other speed schedule chosen) and the engines set for optimum TSFC. Thus, as fuel burns off, a 'creep' climb will develop, with both the airframe and the engines at their optimum.
Regards,
Old Smokey