Firstly, the "shallow approach to a low hover" scenario involves the helicopter making a shallow descent to hover at a low altitude. While the approach is shallow, hovering at a low altitude typically results in significant rotor downwash due to the proximity of the rotor blades to the ground.

Secondly, the "shallow approach to a high hover" case means that the helicopter descends shallowly but then hovers at a higher altitude. Hovering at a higher altitude can indeed reduce the impact of rotor downwash on the ground compared to hovering at a lower altitude. However, rotor downwash still exists.

Thirdly, the "land in the first third or half of the runway and ground taxi in" option suggests landing the helicopter in the first portion of the runway and then ground taxiing to the desired location. By doing so, the helicopter minimizes the time spent in a hover close to the ground, thereby reducing the impact of rotor downwash on the surface. This approach allows the helicopter to use forward speed to assist in the dissipation of rotor downwash as the helicopter transitions from flight to ground taxiing.

Lastly, the "steep approach to a high hover" scenario involves a steeper descent angle to reach a high hover altitude. While this approach may reduce the time spent hovering close to the ground, a steep descent angle can increase the intensity of rotor downwash during the approach phase.

Considering these options, "Land in the first third or half of the runway and ground taxi in" appears to be the most suitable choice for minimizing rotor downwash. It emphasizes landing quickly and utilizing ground taxiing to reduce the impact of rotor downwash on the ground, which can be especially beneficial in areas where rotor downwash could cause hazards or disturbances.