I wouldn't say it's "improper" form. And I wouldn't say they "should" either. I mean, I have watched guys shoot 45 baby vegas X's with a bow setup this way. As a human, your hand and arm has to go along side the riser of the bow, it cannot be centered like on a machine. There is no way for a human to replicate "perfect form" in my opinion. The best you can do is be consistent. It doesn't matter if the arrow goes end over end on the way to the target, as long as you and your equipment do the same thing every time, they only go where you point them. And you are correct, I would not allow another person to set centershot for me. I also wouldn't allow someone to set nock height, timing, or tiller for me either, because it needs to be based on how the bow sets in my hand. A perfect example is this.......... You can set timing on a bow to be perfect. Now take your stabilizer off (typically a longer target type stabilizer). What does that do to your timing? It makes the bow sit different in your hand, and quite commonly can affect the timing. The same can easily happen when a bow is sitting in someone elses hand. That is why I only set a base line for timing on a draw board, to really tune it I'll creep the bow. Same thing, it's gotta be tuned to me. When I set centershot, I start with an arrow that is spined correctly based on OnTarget2 data. Sometimes I'll shoot a really stiff arrow though to pick up on arrow diameter advantages when shooting paper/3D. I then read the manufacturer recommended centershot, and start my rest there. I shoot through paper from about 8 yards away to make sure it's not doing anything really funky. Then I walkback tune to make sure it's dropping straight. I'll typically walkback tune on a huge bale at the club I belong to, starting at 20 yards and going back to 50 (on a 3D/hunting setup, further on a field/FITA setup). If I get a clean walkback tune, for a 3D bow I'm done. For a hunting bow, I may/may not decide to throw a fixed blade head on the bow to further tune for fixed heads to hit the same place as field points. To be honest, I really think people overthink trying to "perfectly tune" their bows. It's not rocket science. If you tune it with some basics, and then work on perfecting your shot........your going to outperform 99% of the shooters out there.
