Flying manually with autothrottle engaged is a big no no for the airlines I've worked for.
I expect 'tis quite true...except for a few operators with L1011 aircraft, where it was (is) allowed, as the autothrottle system(s) on that airplane are first rate, without exception.
I would suggest however....one hand on the throttles, as you never know when a burst of thrust might be needed, on very short order...to save your bacon.