I think this topic is overstated.
When you are driving, your foot is on the accelerator. If something happens in front of you it takes a heartbeat to move the foot to the brake pedal.
If you are sitting concentrating with your hand on your knee and some malarkey with the engine goes down then you should easily react in time.
Where this goes wrong is generally in the MEH world where the PF is far too involved in an OEI diagnosis and actions and his brain does not reconfigure quick enough to the second engine failing.
In the commercial world we at least get he opportunity in FSTDs to practise this and improve the startle effect. Sadly in the private world of robbies etc, they do not get this kind of opportunity.
Therefore if I was a Robbie drive I would try to keep my hand on or near the collective.