Always button a double breasted all the way?

I'm not sure where this "rule" started, but it certainly isn't observed all that much. One could argue, "Jovan, are you going to take your style cues from the average man?" Well, considering the average man either buttons everything up even when it shouldn't be (I observe guys doing this on their two or three button jackets all the time) or just ignores those pesky little buttons altogether, I think I'm in good company with these stylish men.

Cary Grant, often considered an arbiter of style and taste. He took some risks back in his day, but this certainly had to be the least controversial of them.

When it came to double breasted jackets, Roger Moore almost always left his bottom button undone and wore 6x2, with a few exceptions to both.

Now, I'm not saying it has to be left unfastened. The great thing here is that you have some flexibility, since the buttons are fastened in a straight line top to bottom as opposed to a single breasted jacket. You can fasten the top only, both, and bottom only. Some might say a couple of those suggestions are akin to sartorial heresy. I say, again, that I'm in good company. A double breasted needn't look "buttoned up" in a negative way. Furthermore, it makes sense in an age where young men and old are starting to rediscover just how fantastic tailored menswear can be and are trying to make it cool again. I think just a little flexibility and less emphasis on looking 100% proper is necessary to keep the double breasted, and tailored menswear in general, relevant. Yet in keeping them modern, we can take some cues from the past in wearing suits with a relaxed attitude.

The Duke of Windsor in one of the many 4x2 suits he was known for, almost always with the lapel rolled down to the bottom.

Fred Astaire, among others, also wore his like this. During the "golden age" of menswear, it was considered stylish in a devil may care sort of way.

In summary, fasten your double breasted suits any damn way you want -- so long as it's not left open while standing up (all that fabric flapping about looks untidy).

A more modern take on the double breasted suit from Black Lapel. Here, the clear advantages of leaving the bottom open are shown.

Jon Cho of Star Trek/Harold and Kumar fame modelling another slim fitting double breasted.

Prince Charles, one of the most well known double breasted suit wearers, usually fastens all his buttons. He also has creased sleeves on many of his suits, adding to the more "old fashioned" look.