"User is Dragging the Transform Gizmo" in Editor

I'm trying to write a custom editor that:

• lines up transform.position with the nearest whole number value

• registers that grid intersection as "in use"

I want to do this automatically, by rounding transform.position, seeing if it has changed, and making any changes if necessary.

BUT

I want to make sure I don't do the rounding operation unless the user is not dragging the object in the editor window. In pseudocode:

 if (!transformGizmoBeingDraggedEvent)
 {
      do the transform sanitation;
      register the gridspace;
      etc.;
 }

Is anything like this possible? The Editor/GUI system still confuses me :P