Un-tucking of the shirt has been acceptable, if not preferable, for a while now.
Leaving a shirt un-tucked does the following things: (1) It says you're relaxed. (2) It aligns you with youth. (3) On shorter men it creates the look of a longer torso, making them look taller or more formidable. (4) On men who have a stocky build it puts less material at the hips, which in turn makes them look more masculine for the reason that it looks less feminine. (5) It is less cumbersome when you're walking, running, or otherwise moving because there's less material jostling around down there. (6) You won't get as sweaty at the waist, pant-line, and crotch, which cleans up your appeal. (7) It allows you to buy a smaller-sized pant or short. (8) It allows you to be lazier because you don't have to do the work of tucking your shirt in, and constantly worrying about how misaligned it has become.
The only time I'd tuck in my shirt is for formal dress, a formal occasion, or if my boss threatens to fire me (and even then I might not). Un-tucking your shirt is freedom! lol