Generally wind the velocity is greater with altitude so when flying downwind flying higher will be quicker and potentially more economical. Going downwind lower is better if you can tolerate the turbulence! The wind forecasts give an idea, for each day, on the amount of shear.
I'm sure a more numerate person than me can work out how far one needs to be cruising downwind before the the cost of extra time (fuel) in the climb is outweighed by savings in the cruise given appropriate assumptions.