Oh my goodness I'm afraid you may have touched on a personal rant. I have so much to say about buttons I might have to post pictures. That trick about insetting the top and bottom buttons on vertical button holes seems like a good one. I've experiences gaping on my plackets sometimes, especially with extended wear. That would probably fix it.
I know that horizontal button holes are often chosen because you can be a little more sloppy with how the buttons get sewn on. I have also seen people draft vertical button holes 1/8" to the inside of center because it will cause pull and "pull" to center. Which I think is what you were saying in the previous post. I have also heard that you should draft vertical button holes 1/8" to the outside of center to allow for the thread under the button. Thus making the center of the button on center instead of the edge of the thread. I think this is more important with big buttons and heavy fabrics. I personally subscribe to the second idea because I would like to have the placket of my shirts not have any pulling. I also have a feeling that it will depend more on the fabric and button combination that anything. Imagine for instances that you are making a coat and the front opening in made with two layers of heavy wool. I can easily imaging loosing 1/8 " in thread it would take to make a button hole in that fabric.
Anyway maybe there will be more to come if I can find some time.