I only left my car over night in the parking garage once. I was staying offsite on Major Blvd, there was a tropical storm on the way, and I asked a parking attendant if it would be okay (he said "yes").
I don't know what the "official" policy is, but I imagine they would want to discourage the practice. Still, I would imagine a few cars get left over night almost every night. (Car trouble, lost keys, drunks taking taxis home, etc.)
If you do it, I would recommend moving your car every day. A car sitting in the same spot, night after night, will definitely get noticed. (Come to the parking garage after CityWalk's 2am closing and you'll see how the few cars left really stick out!)