For optimum control of your vehicle you should always have both hands on the steering wheel.
Acceptable are the 10/2, 9/3 or 8/4 position.
Now that all cars have power steering, it is not really needed anymore to keep the thumbs out of your steering wheel. But I consider it still a good practise to keep your thumbs in a safe position. You never know.

10 o'clock / 2 o'clock

9 o'clock / 3 o'clock

8 o'clock / 4 o'clock



The recommendation not to have your thumbs inside the stering wheel stems from times without power steering. Back then forces working on the tires (like hitting a rock) could easily spin the steering wheel and hurt your thumb(s). In rough terrain broken thumbs were not uncommon.