I agree with Michael, you do not want the overmat to cover too much of the photo. Most precut mats are cut to 1/2 inch smaller than image size. In other words a mat that allows an 8x10 photo to placed in an 11x14 inch frame will have an opening size of 7-1/2 x 9-1/2. That results in 1/4 inch of your image being covered on all sides. Our standard size mats are cut 1/4 inch smaller so they only overlap 1/8 inch unless than image size is greater than 18x24 then we still do 1/2". However, if you order custom from us you can specify any size you want. I would like to make our standard sizes smaller, however, we have many customers that do not have the kind of measuring and aligning skills that a photographer doing his own prints in a darkroom will have.
I also agree with Michael about consideration of the photo paper border. However, showing the border does make it easier to show 100% of the image and many photographers like to use the border area for their signature. The importance of this will vary based on the image and the photo paper it is printed on. You also want to avoid using white mat that is brighter than the whitest point in the image for the same reason that Michael mentioned about the paper border.