As a member of my hospital's Trauma team I have seen one patient in a dozen years with injuries to his thumb from an air bag deployment. I believe he was an older dude that drove with one hand on the center section of the steering wheel (my dad drove like that).
In race school (road racing) we where taught to position the arms so there is a slight bend at the elbows with the hands at a position that will allow as much rotation of the wheel without getting your arms crossed up. Nascar drivers generally don't have that much steering input and usually only steer left...
Off roaders keep their thumbs from wrapping around the wheel due to kick back.
With the relative ease of today steering systems I think 10 and 2 or 9 and 3 offer the best comfort and reaction ability combination. YMMV!
Of course these days both hands on the wheel would be a HUGE increase in most drivers' safety!