If you do it vertically and even, then the effect is to open the room and let the light and the visual be the point.
If you get fancy, then you will be drawing attentio to the mirrors themselves, not the light or the reflected view.
It s a design issue . . . For me, I'd stay with vertical and even so you can position furniture and accessories to wrap the feel of the room. Remember, we are not in the 80's anymore!