That's the way suits are supposed to be worn. If you button the bottom button it bunches up when you sit down. It's mainly for that reason that we don't button the bottom. When brothers get on the platform some I know do button them up then unbutton them when they get off.
You have your jacket buttoned when you sit???
To answer your question, I don't know, I just did it cause that's how every other guy I saw wore their jackets. I only button the top button while I'm walking around, unbutton it when I sit. I didn't have a father to tell me the right way or not.