Of course, takeoff and climb are typically at full power but to reach cruising altitudes for a single engine airplane doesn't take very long. It's a similar concept to a car on a highway onramp, except that airplanes actually get more efficient at higher altitudes.

It factors into overall consumption but it doesn't really blow the whole equation for efficiency. Pilots in training do takeoffs and landings on repeat for hours at a stretch between refueling.