I frame 10.5 inch squares (11x11 nominal) in a vertical 16x20 frame "optically centered." But looking at that for 12x12, optically centered or not, I find the sides too narrow. I would probably use an 18x20 or 18 x 22 frame.
Russel Cottrell has a page about the topic with an interactive display. He's updated it since I last saw it, and the final frame drawing seems a bit weird (breaks in the corners), but the basics work. Vertical centering tends to make the top border seem top heavy. Top and sides equal can make the bottom border overly heavy, depending on the specific dimensions. Optical centering places the print just slightly above the vertical center to place a bit of weight on the bottom. I have framed a couple of my shots square, but prefer the vertical rectangle.
A sample here
Last edited by DWThomas; 12-24-2013 at 04:53 PM. Click to view previous post history.