I am going to go against the grain here a bit. In a SD situation at home, my office or out in public in town I will not fire any warning shots.
However, there are instances where I would. Example, at deer camp we have had a lot of stuff come up missing (stolen). From game cameras, deer stands, wiring, battery bank, you name it, if it was of value they have pretty much helped themselves to it. They are clearly tresspassing, have cut fence and are entering property which they are clearly not supposed to be on.
Now if I happen to go through the locked gate some time and I see a 4 wheeler heading towards the place where they like to enter the property, I seriously might take some "warning shots" depending on the exact circumstances. I would not have to worry about hitting innocents, or whether I could call back the bullet or any of that type of thing. I have zero tolerance for folks who help themselves to others property.
So I can't say that there is never a time when warning shots would not be used by me. If you disagree, that's your call, but I know the layout of this area, and the history of folks who frequent it illegally. "Warning" them would definately be in order in my view.