the prevailing wisdom is:
always = middle button
sometimes = top button
Perhaps though, with the hard-roll "true" 3 button style like Eddie is wearing, the jackets were worn with just the top button done and not the middle? I've mostly seen them either both done up or just middle. edit: found a pic of Pete T with just top button done. Steve with the middle button. Ronnie with both done up.
As a side note, I'm re-reading The Soul Stylists by Hewitt...