Tankering fuel is full of pitfalls, not least the difficulty in finding a simple price differential/distance/flight level formula.
Another consideration is the likelihood of wing ice accumulation if you land with a lot of fuel in your wing tanks. Destination weather or flight through iceing conditions in the latter stage of the flight can result in rapid and heavy build-up of ice. If you are landing in foggy but frost free airport (hence no de-icing equipment) at a temp of a few degrees above freezing, it will be a very long wait before the aircraft can make a subsequent take.