You will be hard pressed to find people in the QH world looking back. Because its pretty **** near impossible to trot a straight line while looking back. Plus is looks sloppy. Certainly there is no rule about it. Just like there is no rule about having a fake tail in, but you will never see a person at a breed show without one. There are plenty of un-spoken rules in all aspects of the horse world. This happens to be one of them.