This advice came a number of years ago from a friend who did a lot of counted cross-stitch. When preparing the piece, press it from the back on a soft surface, such as a towel. This allows the threads to stand away from the cloth and be more prominent rather than disappear into that fabric, whatever that might be (cotton or aida cloth). She did not advise putting glass over it--same reason we are encouraged to not store quilts in plastic or something that won't "breathe". If you feel you need the protection of glass over it, put a spacer between the piece and the glass so the threads aren't confined so tightly and gives the fabric a bit of "breathing" room.