That is a stunning quilt, battle axe. I also prefer the borders to go up in width, but I have seen some where a narrower border is in the center of the border. It would add more interest imo if each border strip was a different width with the outermost being the widest. If the first two borders are relatively narrow 1 1/2 and 2 1/2 (for example) then the last border could be wider 4" (for example) and I don't think it would overpower the 6" block.
When I get to the border stage, I lay out the fabrics and fold and smoosh until it feels right to me. It is generally an intuitive process.