In the earliest comics (1965ish and before), when he flew, he had one arm extended and one arm folded, below his torso, almost like a “swimming” position, hands in a fist. I took it as a visual representation of “swimming” through the air. When he “hovered” in the air, his arms often looked like he was “treading water.”
The 1950s TV show portrayed him flying with both arm extended straight ahead of him (no fist, palms down) – undoubtedly, because George Reeves was on a board and couldn’t have one hand lower than his torso. But it also gave a sort of aerodynamic look, like he had shot himself into position with a mighty leap. (How he turned around to land feet first is beyond the scope of this essay.)
After that, I think the artists or movie-makers did whatever they wanted. My recollection is that the Christopher Reeve movies involve different arm positions, for variety and based on what looks good in any given shot. Certainly, by the LOIS AND CLARK TV show, he can just hover in the air when he’s sleeping.