I don't or wouldn't pony either; the reason being, that not only do you endanger your horse, but the horse that is ponying the newly ridden horse, should the horse being ridden for the first time decide to blow up. There would be no way the rider of the ponying horse could get out of the way, or get rid of the lead rope quickly enough to totally prevent some kind of wreck from happening; both horses and riders could be hurt, instead of just one horse or rider.
Now I have had a handler on the ground lead a horse for the first actual ride, but I don't as a general rule any more. If you have taught the horse how to flex, and yield his hips, and he knows how to move off of clicks and kisses from the ground, getting him to move undersaddle isn't going to be rocket science. I can generally get a horse to w/t/c the first ride, but mainly focus on bending, and stops, and keeping him calm and relaxed, how fast we go depends on how focused he is on me.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."