Very nice video but I have to agree about not tacking with an X. I am not a dry mounting expert but I just attended a mounting class at the West Coast Art & Frame convention taught by Chris A. Pacschke. She is the leading expert on mounting for the picture framing industry and has been consulted by various paper and mounting equipment manufacturers and served on preservation boards with people like Wilhelm of Wilhelm Imaging Research.
She is the author of the book “The Mounting and Laminating Handbook” which I highly recommend. It is considered to be a mounting bible for professional custom picture framers.
This is a quote from her book, page 87 of the 3rd edition: “Tacking must be done in one spot only, along the end or side of the item to be mounted. Do not tack along an entire edge, at all four corners, in opposite corners or in the center. This could inhibit the paper expansion/contraction when adjusting to temperature and humidity changes while being mounted and create permanent wrinkles or creases in the mounting”
She has a great website with lots of articles she has written for framing magazines:
Click “Library” button in the lower left side to go to her archive of articles.