The issue of guarding the controls is also related to the knowledge about automatic systems in general – how automatic is automatic, and what are the design and certification assumptions?
Thus for older aircraft with ‘fail hard’ autopilots it was definitely hands on.
For more modern ‘fail passive’ (fail soft) systems then hands off is quite acceptable in the cruise. In the event of a failure, the certification requirements should limit the pitch excursion to approx 1 deg / sec (300 ft alt in 4 sec) and less than 15 deg / sec roll (60 deg AOB in 4 sec); most autopilots will disconnect automatically. The cert assumption is that the crew will be in the loop within 4 sec. With some aircraft there has to be a balance between the crew deciding to be hands on and them not fighting the autopilot or attempting to overpower the system (with potentially hazardous results).
During the approach and landing, whilst the cert requirements are tighter (crew involved within 2 sec) it is more sensible to guard the controls. Some aircraft specifically require the crew to be hands on i.e. for the Avro RJ to accommodate extreme conditions, hands on (follow through) is required to damp/prevent a low amplitude oscillation, which the cert standard would not allow, but I doubt if it would ever be seen in service.
For landing with fail operational systems then hands off would be more appropriate as the auto-land (even after the first failure) will perform far better than any crew i.e. the crew are more likely to fail / screw up than the system.
Personally I choose to follow through on the controls whenever possible; for those systems that back drive the controls the pilot gets a feel for how hard the auto pilot is working and whether it is in touch with the aircraft motion; a bit like supervising a student pilot.
Now apply that principle to other automatic systems; how do you maintain hands on for an automatic fuel transfer system? Is the system fully automatic or only just automatic?
With apologies to co-pilots and autopilots:
“An autopilot is like a co-pilot, but never learns”
“A co-pilot is like an autopilot, but can forget”
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Airspeed and Upwardness