Initially I couldn't believe that such a basic question was being asked and still not answered, but I always like to have documented references ready before I post, and to my surprise, I couldn't find one now. I'm surprised and humbled.
However, I very clearly remember in my days as a trainee, it was clearly explained (with FCTM references) that the levers must be moved slowly enough, so that actual N1 (green needle) closely follows the lever position (blue circle) on the EWD.
Safety First issue 27, has some useful info.
Anecdotally, I've had dodgy engines that from 50% (stabilised) to FLX had enough of differential thrust to require full rudder deflection. Surprisingly even after writing it up, the reply was that the acceleration on both engines was "within limits".