[Anthony]

The valves, as positioned in the head, are not parallel with the bores of the block, but are slightly tilted such that the valve stem tips are tilted slightly towards the center of the engine. As the valve is pushed down, the top edge of the valve will hit the top of a flat top piston first, and therefore require that the valve relief posckets of the piston are situated at the tops of the pistons not the bottoms.

Now if you are running a reverse flow head, like on '60's era F1/indy engines, where the intake runners were on the sides of the block, and the exhaust came out near the top center of the engine, you could run true 180 degree headers for maximum exhaust efficiency, and in this case, you would need to put the heads on upside down, and therefore, position the pistons with the valve reliefs down like in your picture.