Rounding position with mouse constant update

I'm making a mario maker style game where everything has to snap to a grid. I want to, while I'm dragging it, have the item snap into place closest to each int.

     void FixedUpdate () {
         Vector2 pos = Input.mousePosition;
         pos.x = Mathf.Round(pos.x);
         pos.y = Mathf.Round(pos.y);
         pos = Camera.main.ScreenToWorldPoint (pos);
         transform.position = pos;
     }

This does not work for me, and I've been trying for hours to fix this. Nothing has helped so far, I've gone through every forum I could find to no avail. Please help!!