I have never known an Airbus FBW autopilot to suddenly drop out by itself* nor have I ever seen one do something weird uncommanded - and we had some pretty old Airbuses. In my experience, they are very reliable and dependable.
If the autopilot is off then obviously one should be grasping the side stick. However, owing to the fly-by-wire, if one was hand flying in smooth air and needed to scratch one’s nose while the other hand was holding the deployed speed brake, say, nothing untoward will happen if the side stick is left for a couple of seconds. Obviously, if one was hand flying a turbulent approach requiring constant and rapid changes to correct the flight path, then a hand should always stay on the side stick.
If the autopilot is on, there is no need although it is a sensible precaution during an autoland or turbulence - one is controlling the flight from the FCU or the FMGS - and the side-stick is locked in neutral anyway.
Holding a locked side stick until FL100 seems unnecessary to me.
*It did drop out when the iPad and its mount fell off the window onto the locked side stick !
