Not "as high as possible" - there is an optimum FL for fuel burn that is weight dependent. Most jets are capable of flying higher than the optimum, so you need to refer to the performance data in the ODM. Usual practice is to try to be within 2000 ft of optimum. Another consideration, especially for crews who regularly fly at high latitudes, is exposure to cosmic radiation - one transpolar sector gives about the same exposure as a chest x-ray.
For short sectors, practicalities as discussed in previous really determine what you can do. Typically, a shorthaul twinjet will take around 25-30 min to climb to FL350, and about the same for descent and approach. For a short sector, it's a good idea from the crew workload aspect to have at least 10 min of cruise; so, for say a 40 min sector, somewhere around the FL250 region is about as high as you will get.
What Denti said is right, but a parabolic flight path gives just about max workload for the crew and for ATC.