Short answer: NEVER!
You're going to find blazers and sport coats with one, two, three and even four buttons. In most modern and updated cuts or styles the mid button will fall about 2 inches above your bellybutton. If at all, this mid button will be the only one that you fasten. The reason being is, the toggle wrap that is formed by fastening this button right above your waist, will create a sense of balance and symmetry between your upper and lower body. It will also, visually, elongate your legs and create The effect of a more athletic and tapered upper body.
Nowadays most blazers, jackets, and sport coats with 2 and 3 buttons are not even designed to fasten other than the mid button. The lower button is merely an element of style and to differentiate a more formal 1 button blazer.
The top button in a 3 button jacket (the 3 button roll) is also a style element that adds an hint of utilitarian flair. Again, not functional but aesthetic.
In bullet points:
-One Button Blazer/Jacket: Worn mostly in formal events and the only button can be and should be fastened at any time while standing up. Unbutton when seated.
-Two Button Blazer/sport coat: Only top button should be fastened in more professional/formal settings. Unbutton when seated.
-Three Button Blazer/Sport Coat: Only mid button should be fastened in professional/formal settings. Never, ever fasten top an bottom buttons. Please. (Btw, 3 button Blazers are NOT all out of style but make sure you pick a "3 button roll" blazer. It will make a huge difference).
-Four Button Blazer/Sport Coat: follow the 3button rule.
-All blazers/Sport Coats can be worn unbuttoned, specially when you're sitting down or when wearing more comfortable and loose slacks.
-Only button your blazer in more professional or formal situations (or if its cold of course).
-make sure your blazer fits properly through the midsection, length, shoulders and sleeve.
If I am confusing you or you have any questions or comments, give us your feedback or stop by the store.