Am I supposed to button up a sports coat at a formal occasion, or am I supposed to keep it open?
I’ve always been told to button them all EXCEPT the bottom button. You can unbutton the coat to sit down, or if you’re in a less formal setting, but when you’re talking to the judge (or doing something else formal), button all but the bottom one.
Opinions will no doubt vary on this.
There is little harm in leaving it unbuttoned. It seems to look better.
You don’t have to button it, but if you like how it looks, by all means do so. But don’t button the bottom one.
If you do go for the buttoned look, make sure to unbutton when you sit down, or you will look ridiculous.
Unbuttoned when you sit, buttoned when you stand. It’s no great sin to walk around with a single-breasted jacket unbuttoned as long as you’re not in a formal situation, although it looks better buttoned IMHO. (Double breasted, no way.) Oh, and if you’re ever going to be a talkng head on CNN or The NewsHour, button it even while you’re sitting down.
–Cliffy
…and sit on the tail to keep the front looking crisp while you’re on-air.
Related tip, because I just saw this again this morning: If you have a new sport jacket and the vent in the back of the jacket is still held together with a stitch or two, go ahead and snip the stitches. They are there just to keep the jacket neat-looking while it’s still on the store rack. Once the jacket’s on you, all the stitches do is say “new jacket!” to anyone who sees you from the rear. 
Oh, and also snip off the brand tag from the wrist!
–Cliffy
I usually have the vent closed on a suit or blazer. I prefer a ventless jacket.
And cut the stitches out of the pockets, lest you have nowhere to put your hands!
Actually, I’d leave them closed. That way you won’t be tempted to use those pockets and ruin the line of the jacket.
With a jacket that’s meant to have an open vent, though, the problem with leaving the stitches in is that the rest of the vent still pops open while the guy walks, and the jacket doesn’t look right. Also, sometimes the stitches themselves are quite obviously only supposed to be temporary, so a noticeable “X” across the vent flaps looks silly.
If you prefer ventless, that’s fine, but then it’s best to buy them or have them made that way.
Exactly what I came in here to say. You can always tell the guy wearing the first suit of his life. He’s the one who kept the brand label on his wrist. The older the guy, the more embarrassed I am for him.
Also, as a practical matter it makes sense to keep the jacket buttoned when you’re outside, lest that sudden breeze splay your jacket flaps and tie all askew. Indoors, I prefer buttoned except when sitting, as it makes a neater overall appearance. But unbuttoned has its moments.
I got this from Queer Eye for the Straight Guy, so weigh it however you please.
For sport coat buttons, assuming there are three of them –
Top: Sometimes
Middle: Always
Bottom: Never
I don’t mean leaving the temporary stitches in. I mean having your tailor stitch the vent closed, converting the back seam to a closed seam all the way down.
It’s simple to do, especially if the jacket is being altered already.
To hijack slightly, the more tailored casual blazers that are popular these days (at least in my neck of the woods) - the kind made of velvet-like material, or corduroy, or other such vanities - look a lot better buttoned than not, imho. They’re meant to emphasize or suggest a wedge-like upper body on men, as opposed to just serving as a jacket. As such, it’s more about the line than about the functionality, and that’s better served by keeping the top or middle button fastened while standing.
Of course, I always button the top button when I’m wearing a blazer, so ymmv.