Here we go:
If your blazer, suit’s jacket or sports coat/jacket is an one-button piece then button up that button.
If it is a two-button one then only fast the top button.
If it is a three-button piece then the middle button should always be fasten. The bottom button always left unbuttoned. The top button could be left unbuttoned or be buttoned up. It is up to you.
Having said that if your jacket, blazer or sport’s coat is a rolled up into a lapel (like the photo above), then this is a strong suggestion to leave it unbuttoned.
If it is a four or five-button piece (seriously?), always leave the bottom button unbuttoned. The top button may be fasten or not.
If it is a double-breasted one, then usually you keep all the buttons fasten.
Another possibility: the bottom button from the outside is left unbuttoned. Look at the photo for a better understanding.
For single-button pieces, no matter the configuration (one-button, two-button, three-button etc) it is better to keep it buttoned up properly as it is illustrated above while standing and unbuttoned completely while sat.
Of course that if you are wearing a jacket’s suit, a blazer or a sports coat/jacket casually, it is an option to leave it unbuttoned while standing. But if it is a black-tie, formal or business event, keep it buttoned up properly at all times while standing. Or at least at pivotal moments, like in an Oscar ceremony during the red carpet and on stage if presenting an award or receiving it.
For double-breasted button pieces it is better to keep it buttoned up even while sat. Having said that, since the introduction of the slim fit suit, sometimes in this case it is totally doable to leave it unbuttoned while sat. And it is because with a slim fit suit there are less fabric hanging. But still…