I believe that the gearing of the steering and the tightness of the course would determine thumb placement. If there are more than 3 turns from lock to lock you may find your thumbs at risk as you seek the next handhold.
Should never have to risk the thumbs on the street surface.
You can't say never to allow risky thumb placement unless the environment is sanitary, ie , a closed circuit where you have flogged the same car around for many laps.
Even the best(arguably) have had their thumbs at risk... Colin MacRae broke a thumb the day he decided to fire Nicky Grist as Co-Pilot
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