Yes, best to sit near the middle of the plane to reduce movement. When you see the wings bending - that's what they are supposed to do. Because they flex - they do not break. Just like a flower or tree in the wind. Sometimes, it can seem that they are moving a LOT (up and down) but they are always flown well within the tested limits. Really, the testing is long and arduous so that they know what will break it and then keep well clear of anything that would.