I had the impression that an aircraft would pretty much only climb during a normal flight (as it burned fuel and became light enough for higher flight levels), then descend only at the end of the flight.
That is generally the case in the real world but there are indeed times it might be advantage to consider decending to get out of strong headwinds/significant turbulence and I've seen flight plans that ask you to do exactly that. However, again in the real world, especially Longhaul and most especially on some of the more congested routes if you descend en-route you may be unable to get clearance to climb back up again to a higher level because another aircraft has joined your route of flight above you and is blocking you.
Most folks game plan is to climb the higher levels when it's economical to go there..and stay there.