When mounting a print it is important to visually isolate the print, placing the print in the centre of a mount creates an optical illusion that the print is not equally spaced at the top and bottom. To overcome this illusion many photographers / framers adopt the technique which invovles placing the print at the Optical Centre.
Below is a link to help you locate this optical centre, the website allows you to enter your photo and matt sizes.
For your inner math geek, there formula is explained.
Obviously mounting of images can also be based on your personal preferences but we suggest you give it a go and see if you think it improves the display of your photos.
Thanks to Ben Knight for sending us this link.