My guess is that it is mainly for the same reason that the cockpit is nearly always positioned in the front of an airplane. The reason for this seems to basically be visibility.
Visibility is probably not a matter of personal safety for Superman - crashing into something might not kill him - but, being a nice guy, he probably wouldn't want to destroy an airplane filled with people in mid-flight. Even if he managed to save them all, it would probably ruin their day. And even when there are no airplanes around, crashing into fifteen geese is likely not that fun either.
My own personal addition to Superman flying headfirst is that it makes more sense anatomically. His head (or, often, his extended arm) is one item, whereas his legs are two separate items. Air resistance, enormous at these speeds, will likely work to pry his legs apart and make them flail every which way. Of course, he is Superman, so he has enough muscle to keep them together, but why get distracted by the unnecessary muscle tension? Just fly head (arm) first, and leave legs in the back to drag.
Superman is also occasionally portrayed with both arms extended. In principle, this makes some sense as well, if the double-arm pose was the water-dynamic pose used by divers.
However, the occasional double-arm pose tends to be closer to this:
Now this one I find rather impractical. The only way extending your arms would make sense is if you A. Keep your hands together, B. keep your hands open and not in fists, and C. tuck your head in. The obvious reason for the less practical pose is that it looks extra cool, but come on Superman. Everyone already knows you're heroic and whatnot.