As said above it depends on how your company work out their cost profile.
Lowest fuel use on a short trip is generally straight up and striaght down without a cruise.
Lowest fuel on a long leg is as high as you can get.
However I believe that the largest cost per hour is still servicing so the height that gives you the highest groundspeed for the day is the cheapest. Generally around highest TAS depending on winds.
Some flight planning programmes help with this. A particularly good one (I am not connected with the company) is PPS by Air Support of Denmark which uses your own specific airframe with the winds of the day. It may give you different routes and heights for the same trip on different days. It also prints a table on your PLOG to show how costs change by going up or down slightly or by changing payload.
MM