Maybe whenever control is turned over to the pilot from the computer...
When the pilot normally "takes control" it is from the Autopilot - nothing to do with the "computer" typically referred to above as in HAL / Control Laws etc. Nobody rated on type
should have an issue in Normal Law manual flying at cruise Alt.
If you mean when there is a significant failure such that the AP drops out AND you get a control law degradation, then yes, a descent is advised. IIRC it is 4000' below the normal "Max Alt" to avoid triggering the (now more sensitive) Stall warning.