Yes, it isn't recommended as it may cause the tires to fail. But, this is very, very rare. The manual recommends all manner of preventative measures and while they're all good ideas they're also not at all necessary.
My personal experience says otherwise.
My Ford owner's manual says otherwise. According to it, the tires will be damaged at a prolonged period, if the car sits at the same spot. Maybe not deformed per se, but damaged. It's recommended that the car should be placed on supports if long-term (say, more than a month) storage is required.