If there is such a thing as a ram effect (which I personally doubt) running with the head backwards will remove it.
Running with the head rearwards might stop dust etc being drawn into the inlet which I would think is probably true at slow speeds. If travelling at speed I would think a low pressure area would build up behind the head so drawing any flying dust etc towards it so allowing it to get sucked down. Maybe not so much as having it blown directly down though
Without the benefit of an aerodynamics engineer, a wind tunnel, a bag of dust and a sacrificial engine I guess we'll have to imagine what is going on.
Personally I'd keep it forward under normal conditions and turn it around for high dust just in case.
However, I suspect regular cleaning of the airfilter is the most important thing.
By the time you account for bends, constrictions, airfilter and the effects of a turbo and intercooler I suspect any ram effect is minimal.
I've filed the groove in the back of mine out a bit so I can run the tube for the diff vents etc down into the top.
I'd be interested to know if any of the pot type snorkels have a dust trap in them. I would have thought it possible to incorprate one.