Us flatlanders rarely use the parking brake, though we should, just to keep the emergency brake properly exercised. Nobody in my town has ever talked about their car slipping out of park. In fact, my experience tells me it's more a problem than good practice.... a few times I left the parking brake on a shared car, only to find the next driver didn't realize it was on and drove the car for some distance with the parking brake engaged.

On the other hand, I've lived a while in San Francisco. Runaway cars are a constant problem and cause a lot of property damage. The law says that if a grade is over 3%, a parked car must have it's wheels curbed. This means that if the car is parked facing uphill or downhill, the wheels must be turned such that it'll immediately hit the curb in the unfortunate event that it would start moving. On the really steep hills, parking is perpendicular to the street.

__________________
95 E320 Cabriolet, 169K