Freed usually falls asleep lying on his back. He feels most comfortable in this sleeping posture and it's a kind of habit influenced by sayings like ‘never turn your back on someone’. When learning how to swordfight this is most probably the sentence he has learnt first and told himself the most - never turn your back on the opponent! An aspect of prime importance, not only for swordfights but when it gets around fighting in general. Well, and in his case this kind of rule managed to sneak into and influence his sleeping position to the point where Freed just doesn't really think about it anymore but usually goes to lie on his back because it seems to be the posture with the most comfort.
When he is with Laxus, it can happen a lot easier that he switches his common position and falls asleep very close to him, perhaps lying on his side and having his face buried against him or resting his head on Laxus' chest as long as the blond and he himself will be comfortable with falling asleep like this. Even then though Freed tends to roll around at some point during the night and end up lying on his back.