If you are going to sell your prints fully framed then you can select any mat borders, equal or unequal, or bottom weighted that you feel most comfortable presenting your artwork with. If you are going to sell them mounted and matted, then you might consider matting them such that the frame size is a standard frame size such as 16x20 or 18x24. Then your customer will have an easier time finding a frame and will not be forced to take your image to a custom framer.