judithsr
Ok, I think I understand what you're asking...I was taught that, when matting an image, it's good to have it evenly centered on both sides of the paper, but then have a bit more space on the bottom showing than the top. It helps weigh the image down. If you want, you can have the image directly centered. Be careful though that more is not showing on the top than the bottom. That'll make the image look like it's falling down.

ishel13
No more than 1/2 inch of the bottom mat should show and usually it is about 1/4 inch of the bottom showing. The mat closest to the artwork acts as a bridge between the artwork and the top or widest mat.

Will M
1/4 inch is nice