i guess it depends on what the goal is graphically. if the game is going to maintain a noticeably cubic grid structure, i think it's only fitting to keep a square sun/moon. but if the goal is to give everything a smoother, more rounded appearance i'd go with a round sun/moon.

as it stands, i like the square structure. it is reminiscent of the "good old days" of gaming and sort of sets the game apart from those that focus so heavily on highly polished graphics rather than depth and experience.