I am a firm believer that warning shots should never be used just to scare people off. If you are going to fire a warning shot, you must be prepared to follow-up with a shot that could kill someone.
I concur, Uncle Buck.
Here's a warning shot situation: you're leaving the mall on your way to your car with your purchases. Three girls about thirteen years old decide it's time to try robbery for fun and profit, and two have knives. Are you faced with an immediate, credible threat of great bodily harm or death? Yeah, but they're thirteen-year-old girls. In that situation, I'd put a round in the asphalt and hope they'd undergo a rapid change of heart.
In my home? Not a chance.