A turbine engine has a comparatively narrow design operating envelope. The main problem looking at height is having to operate at lower rpm at the lower levels to keep the EAS under control.
If you look at the fuel flows against height you will note that the lower levels have fairly horrendous burns compared to the higher.. although, generally, at the extreme limits for an aircraft, the very high levels will show a further slight decrease.
Hence, it is usually better fuel burn policy to climb and then descend, with a minimal cruise.
Apart from that, it is a great mental exercise (and good fun) to fly a (fair weather, by day, ferry or freight) short sector with no fancy JB gear (ie only the DME for profile planning and monitoring), straight up and then straight down, especially if the runways are aligned and the descent runs straight into the ILS. Beers are on him (or her, these days) who has to spin up before 1500 feet. Great fun over about an 80-90 nm sector.