Well, I will tell you a very simple rule:
'From Up to Down - Sometimes, Always, Never'.
Yes, that is all!
Most of the modern suits have a single-breasted jacket.
So, when you are wearing a single-breasted one-button suit the rule is: 'When standing you must button the only button. When sitting - you must unbutton'.
For a single-breasted two-button suit jacket the rule is: 'When standing you must button the top button. When sitting - you must unbutton it. The second button should be never buttoned'.
And with a single-breasted three-button jacket, you should remember: 'When standing always button the middle button and if you wish - button the upper one, too. Unbutton when sitting and never button the bottom button'.
Double-breasted jackets are almost always worn buttoned. The rule is very simple: 'Button all the buttons that have working buttonholes'.
If you keep these rules, you can be sure that you will get the best of your perfectly fitting men's suit. It will make you look stylish and elegant and will help you feel comfortable in any situation - both standing and sitting.