Mike,
The mounting corners are acid-free clear corners you use to hold down the... uhm... corners of the print. As long as the print is relatively flat, the mounting corners will only hold it in place, while the pressure against the mat and glass will flattent the print out. The main advantage of mounting corners is archivability & reversability, you can take the print out whenever you wish, no damage done.
BTW, I know that there is a debate over the superiority of dry-mounting vs photo corners, and I do not wish to start it over (however, the frame shop where I work does not dry-mount photos/ limited edition art, just to be safe).