It's true, there is definitely a movement in men's fashion right now inspiring men to mix casual/sportswear with tailored/formal wear in the same outfit to create an "in-between" look comfortable and appropriate for most occasions. This is not a bad thing, I often do it myself. If pulled off properly, it can be a fresh twist on some of your classic pieces. Example:
photo via GQ.com
This "in-between" look can either be accomplished by adding some formal pieces to a casual look (add a waistcoat or blazer to your usual shirt + jeans look) or adding more casual elements to a "dressier" look (throwing on a pair of classic sneakers with a suit, or throwing a vintage denim jacket over your shirt + tie look). Casual-ling up a look, however, is not accomplished by simply leaving your shirt untucked. I see so many men, especially in the nightlife scene, wearing the same shirts they wear during the day, but because they are at "play" and not at work, they change their trousers for jeans, lose the tie and leave their shirts untucked - with shirt tails ballooning past their back pockets.
An untucked shirt does not make a look more casual, just more sloppy, bordering immature. So today's tip is quite simple: when in doubt, tuck your shirt in! (Yes, even when wearing jeans).
With that said, not all shirts need to be tucked in, but you should know the difference. The vast majority of button-down "dress" shirts are long through the body and are meant to be tucked-in (hence the extra fabric in the tail which prevents the shirt from becoming untucked with your day-to-day movement). There are other shirts, however, which are meant to be left untucked - these are shorter, slimmer cut and hit just past your hips (see cuts by Band of Outsiders or American Apparel).
One last thing...If you are wearing a tie, a waistcoat/vest, a cardigan, a v-neck, or basically any other kind of sweater/layer over your shirt (other than outerwear), ALWAYS tuck your shirt in. The shirt ends poking out from under your sweater are not flattering.
Yours in style, DT