You have to have the music in YOur head and heart.
...so it's a good thing to have the piece memorised aurally and if possible photographically. Physical/finger memory is the least reliable of all, especially in Bach pieces - one wrong fingering can throw you off SO bad!
I avoid memory performance for the very reasons outlined above. However, I commit most pieces I work on 'close to' memory otherwise I would not be able to play them effectively.