Constant in the sense that it is never FL60 or FL50 or FL120, but always FL180 unless FL180 is unusable because of low pressure. When ATC issues a clearance to an altitude from a flight level they are required to state the local altimeter setting and if I don't get one I will ask and I change the altimeter setting passing 17,000' on the way up and passing FL190 on the way down and I never descend or climb the last 1,000' at 2,000' per minute.