Finish a rotation with cube face up.

Hi there,

I have created an grid of cubes

And I have the following script that rotates individual cubes when the mouse is pressed down and dragged across:

 var rotationSpeed = 10.0;
 var lerpSpeed = 2.0;
 
 private var speed = new Vector3();
 private var avgSpeed = new Vector3();
 private var dragging = false;
 private var targetSpeedX = new Vector3();
 
 function OnMouseOver() 
 {
     dragging = true;
 }
 
 function OnMouseExit() 
 {
     dragging = false;
 }
 
 
 function Update () 
 {
 
     if (Input.GetMouseButton(0) && dragging) {
         speed = new Vector3(-Input.GetAxis ("Mouse X"), Input.GetAxis("Mouse Y"), 0);
         avgSpeed = Vector3.Lerp(avgSpeed,speed,Time.deltaTime * 5);
     } else {
         if (dragging) {
             speed = avgSpeed;
               dragging = false;
           
         }
         var i = Time.deltaTime * lerpSpeed;
         speed = Vector3.Lerp( speed, Vector3.zero, i);   
     }
 
   transform.Rotate(Vector3.forward, speed.x * rotationSpeed, Space.World);
 }

However, once the cubes have finished rotating the end up something like this:

So my question is, whats the best method to use, so that when the cube finishes rotating it returns to it's original position, i.e face up.

Thanks