Just a reminder: the chance to successfully overmeld grade V materia already bottoms out at 17%/10%/7%/5%. It doesn't go any lower.

If you are buying your materia on the market and Craftsman's Cunning Materia IX are cheaper than Cunning V/VII (on my server, Cunning IX materia have been hovering around 50%-75% the price of the others), just use those in your new jewelry.