An interesting range of discussions that this thread seems to have thrown up, based on the OP's discovery of aches in his upper arm and triceps while driving with the steering held religiously at the 10-2 position.
Allow me to put forward some points and thoughts:
1. 10-2, 9-3 and 8-4 are all valid positions to hold the steering wheel. Each has its advantages and disadvantages, but 9-3 has the least disadvantages as compared to the other positions, and therefore wins by a comfortable margin.
2. We started off by holding the steering wheel in the 10-2 position, till the '70s or so. The steering wheel was large diameter to provide mechanical advantage in a non-PS system controlling a heavy car, and trying to hold it in the 9-3 position meant a certain amount of discomfort because the arms were splayed beyond shoulder width. We were also sitting pretty close to the steering, usually with our chest 6"-8" away from the central boss of the wheel - this allowed the torso muscles to come into action to help turn the wheel too.
3. We were also taught to 'feed' the steering wheel from one grip to another while making a turn, releasing the grip on one hand and letting the wheel slide through, reestablishing grip while the other palm was released to slide back to its original position. This was due to the fact that the number of turns required to turn the wheels was large - usually >4 turns from lock to lock. Crossing the arms while turning the steering wheel was a strict no-no.
4. With the advent of power steering, the wheel became smaller in diameter, and lighter to turn. We did not need to sit so close to the wheel (muscle power requirement being minimised with power steering) - but many cabbies still sit up close to the steering due to antiquated training received from old-timers. Also, the 9-3 position became a more natural one because at 10-2 the arms were closer together and caused discomfort while sitting back from the steering wheel.
5. The number of steering wheel turns from lock to lock reduced in new generation cars, to around 2-3 turns. Most range of vehicular movements could be controlled with a maximum of 3/4th turn of the steering in either direction - hence, no more need to slide/'feed' the steering through from palm to palm. The general rule now is to maintain one palm grip all the way through turning the steering through 270 degrees. The left hand is used to steer to the right, and right hand to turn the front wheels left. The 9-3 position (as well as the 8-4 position) allow this, but not the 10-2 position.
6. 10-2 made us sit close to the steering wheel to maintain a comfortable position. Repeated crash tests showed that with airbag-equipped cars, a minimum distance of 10" was required to allow the exploding airbag to deploy fully before the head strikes it. Too close, and the head/chest would be struck by the exploding (rapidly expanding) airbag, causing injury and even death. This, plus the likelihood of injury to forearms being in the way of an exploding airbag, made 10-2 an undesirable position to hold the wheel (though injury to arms is of much lesser concern than injury to head/chest). In a non-airbag-equipped car, 10-2 may be acceptable, except for point #5.
7. 8-4 upends the palms, causing stress to the wrists while maintaining the posture for prolonged periods. By all means, maintain 8-4 if you must for short periods - but over a longer period, some wrist pain can be expected. 9-3 keeps the wrists in a more natural & unstressed position, so less chances of skeletal pain after a long session of driving.
8. 9-3 also allows the steering mounted control stalks to remain within easy reach of the fingers. If you inspect a car from the '70s or '80s, you'll find many of them had the control stalks angled further upwards to allow easy access when holding the wheel at 10-2. Today's cars have the stalks angled to allow easy access when holding the wheel at 9-3.
9. For those who love to drape their left hand over the gear knob and think the position 'looks cool', allow me to remind you of the first rule of being in the driver's seat -
both hands on wheel, both eyes on road. Moving the left hand for a second to shift gears is all that is ideally allowed (for your own safety, please park the car to address that inexorable itch in the nether regions
). You don't drape your left leg over the clutch pedal when not using it, so why should you give similar treatment to the gear lever?
10. Those interesting combinations like 9-5, 12-6 and whatever else - please refrain from using them for your own safety.