From the standpoint of eliminating dead spots and keeping water circulating throughout, I would keep the back corners filled with rock, tight to the glass. You have an angled overflow in your back left (which is good) so I would glue rock to the overflow wall on that side and then create ledges coming off that. Then duplicate the back right corner to do the same thing (but maybe smaller). This will keep anything from settling in the far back corners, making the back essentially rounded so water can naturally flow in a circular pattern.
The pic below shows what I did with mine (using a fake rock wall, but you can use natural rockwork and duplicate it). By carrying a shallow ledge across the top back (and painting the back black) it makes it look like a deep cave which will give the appearance of more depth (front to back) yet not cover the middle rockwork. It also will give a small ledge all across the back to put other corals on, whereas otherwise it would just be wasted space (which is real estate you can't afford to lose with such a narrow (front to back) tank.