I've just found this at
http://www.faa.gov/avr/afs/acs/61-107.txt
(1) The efficiency of the jet engine at high altitudes
is the primary reason for operating in the high-altitude
environment. The specific fuel consumption of jet engines
decreases as the outside air temperature decreases for constant
revolutions per minute (RPM) and TAS. Thus, by flying at a high
altitude, the pilot is able to operate at flight levels where
fuel economy is best and with the most advantageous cruise speed.
For efficiency, jet aircraft are typically operated at high
altitudes where cruise is usually very close to RPM or exhaust
gas temperature limits.