If when you make photographs you want every square millimeter of your print to count as an integral and necessary part of the picture, then you would never want the overmat to cover any part of the picture space.
I put it this way: As a photographer I am responsible for every square millimeter of the picture space, the same way a composer is responsible for every note. And so I do not want to cover any part of the picture that I saw on the ground glass.
And I do not recommend ever allowing the white photo paper to be visible around the photograph. The reflectance of the paper, since it matches the reflectance of the photograph and will be of the brightest reflectance, will distract the viewer''s eye from the photograph.
So, when overmatting, either leave a border of mat board, which is of lesser reflectance than the photograph, and therefore not distracting at all, or have the overmat come exactly to the edge of the print.
Because as photographers we do this ourselvexs, at our company Lodima Archival Materials (www.lodimaarchivalmaterials.com), we custom-cut all of the overmats that we sell.
Michael A. Smith