The best practice when backing up a vehicle is to use mirrors to supplement looking out the rear window. While it's important to be aware of your surroundings and follow safe backing techniques, prioritizing looking out the rear window gives you the best view of what's directly behind you. Keeping both hands on the steering wheel while backing straight is also a good practice for control, but visually checking your surroundings remains critical. Therefore, the answer would be:
Use mirrors to supplement looking out the rear window.