I wouldn't move. Besides the fact that there could be passengers who have a ticket but no seat assignment, there could be non-revs that eventually get assigned those seats as well.

I would wait till you get to the gate - you can talk to the gate agent just before boarding begins. They'll have a pretty good idea of what the cabin layout will look like and how many non-revs they'll be assigning seats to. Worst case, you can always look around shortly before or after takeoff and move back if you see an empty set of seats.