A woman at a local store told me that the iPad Air Smart Case, black, could stain my iPad Air which is white. She said that the material on the inside of the flap could stain my iPad. Whether it's original or an OEM product. Can it really stain my iPad? I'm looking for a case wuth a sleep/wake button that will not stain my iPad. What is the case material that I should not buy because it stains the iPad?