lol.... the proper way really is top to bottom, missing however many buttons you want to leave undone on the top.
Anyone who went to a private school, etc., will tell you this.
Why does it matter? I suppose you have more control at the top than the bottom, initially, and as the buttons get done up, the bottom ones become more aligned and easier to do. It's also easier to button correctly. I guess it is marginally more efficient, over a period of time.
I'm guessing some people naturally do bottom to top, because they are used to "zipping up". But do these people undo bottom to top also?
This is serious business.