I haven't read many accounts of ACW soldiers refusing to fire or intentionally firing into the air, and those ideas don't turn up consistently in their letters or diaries, at least that I have seen or am aware of. Have read ACW soldiers speak of how they could see so little along the firing line that they never really knew whether they hit anyone or not. When you do read of shots at individuals its usually along the skirmish line or picket line.
In the typical situation along the firing line, with all the smoke and confusion, if someone didn't want to kill they could easily intentionally miss if they wanted to without anyone noticing; however, I would imagine that while in the midst of battle most soldiers were much more concerned about their own safety than going out of their way to kill or not kill.
One account I have on hand:
Men in battle will act very differently; some become greatly excited, others remain perfectly cool. One of the boys in my rear was sitting flat on the ground and discharged his piece in the air at an angle of fort-five degrees, as fast as he could load. "Why do you shoot in the air?" I asked him. "To scare 'em," he replied. He was a pious young man, and the true reason why he did not shoot at the enemy direct, was because of his conscientious scruples on the subject. What struck me as being peculiar was that some of the boys swore energetically, who never before were heard to utter an oath.
- Corporal Henry C. Meyer with the 148th Pennsylvania Infantry at Gettysburg. Meyer,
Civil War Experiences....
Major Samuel H. M. Byers of the 5th Iowa Infantry at Champion Hill:
I could not see far to left or right, the smoke of battle was covering everything. I saw bodies of our men lying near me without knowing who they were, though some of them were my messmates in the morning. The Rebels in front we could not see at all. We simply fired at their lines by guess, and occasionally the blaze of their guns showed exactly where they stood. They kept their line like a wall of fire. When I fired my first shot I had resolved to aim at somebody or something as long as I could see, and a dozen times I tired to bring down an officer I dimly saw on a gray horse before me.
http://www.battleofchampionhill.org/byers.htm