USE TWO HANDS
I suppose if we followed that instructor's advice, it would insure that at least we hold the steering wheel with two hands. Many people seem as though they can't be bothered to. Modern cars with advanced suspensions shield us from the road; they almost tempt us to be lazy. But you can't be fooled by this calm appearance. Cars appear stable when driving along in a straight line. However, when someone near you loses control of their car, and you are forced, within less than 1/2 second, to take an evasive maneuver, the real test of a car's stability comes into question (remember the famous Suzuki Samurai tests?).
BUT WHAT DOES IT MATTER WHERE I HOLD THE WHEEL?
1. It can save your life in an accident situation.
2. You need to obtain the maximum range of motion by holding the steering wheel at opposite sides. This cannot be achieved with one hand, no matter how cool you think it looks.
3. You need to keep at least one hand at its original position to insure that you know when your wheels are pointed straight.
4. Your fingers are closer to the horn, in case you need to use it
5. Your fingers are near the directionals, making it more likely that you will use them.
6. If you have American airbags, your arms will not get ripped open.
7. You can snuggle with your honey at red lights and when you're parked.
8. I won't assume you're a bonehead or a dangerous fool when I pass you