It's all about gravity and that
well if you're genuinely interested...

If your suits hangs well you may (/may/ mind) be able to get away with just doing up the top button and the rest of the front will hang with the buttons in line with the holes. This only works on a 2 button suit. It may hang fine, but it looks rubbish in a 3 buttoned suit, and one button... well... it's a bit all or nothing there.

If you leave the top button undone the weight of the coat naturally pulls down and back which causes gaping at the top button, and an unsightly gap at the back of the collar (as the whole coat has shifted back). It also droops down over the shoulders (damn you gravity!) and a) makes your coat look too big because your shoulders don't go all the way out to the top of the sleeves and b) makes the sleeves hang a bit longer than they should, thus covering too much of your shirt cuff.

You might (and again, this is fairly tenuous) be able to get away with it if you're Mr Strong from the Mr Men and have perfectly square shoulders. If your shoulders are perfectly level, then it might not droop down. I haven't' tested this, though, as I don't know Mr Strong.

Oh, and make sure you're the right shape for the number of buttons too. (the photo of Alexei Sale reminded me - Regardless of which buttons he's done up, I bet when he's not doing comedy he wears a 2 buttoned suit, which suits his shape better).
(Benjimawoo, Fri 5 Oct 2012, 20:07, Reply)