Only if you are flying exotic aircraft long distances at high(er) altitudes does slowing down possibly make any sense. The best way of saving money is to fly at a level that will give you the best possible groundspeed, whether that is at FL50 or FL250 (or more for the posher types). The time taken for climbs and ATC interference in descents wrecks meaningful fuel savings gained by flying at optimal speeds. The biggest savings are gained by “directs” and short visual approaches at destination.