I did what you suggested. Created a custom size, 8x8, with the 3880 margins of .12". Then I enlarged the canvas by .25" all around. Pushed /and didn't push the "scale to media" button and I got an image with a .25" uneven margin all around.
Previously I tried just to increase the size of the image by .25". I've not been able to get a boarderless print.
Lastly I printed a 7 x 7 image on an 8.5x 8.5 media, pushed the "scale" button, and got a uneven boarder on all sides ranging from 1/4 to 1/2".
Lastly, lastly, I created a custom size w/o margins and an image with a .25"margin and got an uneven boarder w/wo pushing the "scale" button.
So my conclusion is no boarderless printing for custom sizes.
But the most acceptable for custom sizes with even boarders is to enlarge the canvas by .25" with the custom size, and NOT push the "scale to media" button.
Thanks to you and Randy for the suggestions.