Which step is unnecessary when parking on a slope?

When parking on a slope, shifting into neutral is unnecessary because it does not provide any security for the vehicle's position. The safe practice involves putting the vehicle in park (for automatic transmissions) or first gear (for manual transmissions) to ensure the car remains stationary.

Setting the parking brake is essential as it locks the rear wheels in place and prevents the vehicle from rolling. Turning off the engine is also a necessary step, as it ensures the vehicle is no longer running and minimizes risks, such as unintended movement or air pollution. Additionally, turning the front wheels toward the curb (or away, depending on the slope's direction) is crucial because it acts as a safeguard that directs the vehicle back into the curb if it were to roll, further preventing potential accidents or damage. Thus, the other steps are integral to proper parking technique, while shifting into neutral does not add any safety benefit in this context.