Just a reply to John Hall.... quote...
I thought there was a thing called 'zoom cruise' where the aircraft is flown at optimum power for the route and would naturally rise as fuel is consumed, then descend at reduced power?
Any idea of allowing an aircraft to settle at its own level must be balanced with the fact that ATC will only give you a set Flight Level which you must maintain.
I haven't heard of Zoom Cruise only Zoom Climb... From Wiki...
Zoom climb - Wikipedia, the free encyclopedia