I suspect this has more to do with the original historic images of Superman from the early covers of Action Comics and the Fleischer Superman animated films. Youtube keeps a nice collection of the Fleischer films on hand with decent video quality.
In the early radio programs and even from the very first Fleischer film Superman (aka The Mad Scientist) Superman was supposedly "leaping" from place to place. But artistically having to show our hero jumping around was stylistically difficult given the film lengths and effort required to make them.
So his leaps would transition from a jumping movement where he would use his running momentum to jump and his hands to point skyward. This often transitioned into a swimmer's pose with his hands at his sides as he would begin to "fly" and gain speed. It was from this position he would streak to his target "like a speeding bullet." Look at the Mad Scientist from 6:36 to 6:40 showing a jumping to flight transition.
- He would often, however, have to deflect materials, bullets, shells, deadly beams of energy and would use his hands to do so. This lead to a pose where he would have at least one of his hands out in front of him to catch or deflect incoming attacks. He would also do this if he were carrying someone to protect them as well.
- This ultimately lead to the later flying scenes where he would be shown flying and artists treated his flying as if it were a form of anti-gravity swimming where he would alter his direction by twisting his body and changing his leading pointing hand.
Today, I would consider it a matter of artistic style, not necessity as to why he might keep his hands out or at his side while flying since the character is so far removed from his origins, artists are likely to assume such poses for appearance sake, not even remembering how or why they came to be.