It amazes me that so few people think to do this. If you are not building a special entertainment room from scratch, the probability is very high that there will be a fireplace, a door, a window, or an opening, precisely where the video display should go. It has happened in my last two houses, in spite of my attempts to avoid it.
So, just go with the flow, and put the system on the room diagonal. Those bothersome side-wall reflections go away, the side channel loudspeakers are a long way from the listeners – if they are in the corners, equalization will be necessary, otherwise move them down the walls a couple of feet or more.
Having lived with this, I have come to really like it. The space behind the TV can hide a subwoofer, or a rack of equipment, or both. It is excellent for a general-purpose entertainment room. Now, I would not hesitate to deliberately design a room in this configuration.