Recently, largely because of this article in the NY Times, there has been a renewed discussion about the necessity (or not) to memorize music for a performance. I think this is a healthy discussion, and one most pianists should have - but the conversation I find myself having most often with my students regarding memorization is their lack of use of the score when practicing!
As important as it is to develop memorization skills, those skills must be constantly nurtured. When you practice without looking at the score, like it or not, small inconsistencies and inaccuracies start to creep into the piece, and before you know it, you're off base.
Looking at the music and reminding yourself, not only of notes, but of expression marks, phrasing, dynamics, articulations, etc. will keep your playing honest, accurate, and make your practicing more satisfying.
So please, use the music!