Fashion-conscious guys I see who have a perfectly flat stomach, always have their shirts tucked into perfectly fitting trousers, with no belt. If the trousers and the shirt are neither too wide nor too tight at the waist, the look is like a million bucks. I'd say this is something to aspire to.
I don't have a washboard abs, so for informal occasion I un-tuck. You can't do it with just any shirt though - shirts designed to be tucked in and stay there, have long front and "tail" that you never want to leave untucked. An un-tuck dress shirt should have much shorter and flatter bottom vs the shirt designed to stay tucked in.
If wearing a suit with a white shirt underneath or a buttoned shirt with pants then I like it tucked in. If a buttoned type shirt is unbuttoned then I wear it untucked. And all basic shirts are always untucked.
it really depends on the look... on how i feel, if i'm trying to go casual, where the focal point of the outfit is, on how accommodating the pieces are (sometimes a tucked top can show lines, some tops seem frumpy untucked, and i may not be able to accessorize well enough to compensate)... probably there are other factors too that i haven't thought of, lol
I would wear them tucked in if i was wearing a pencil skirt.. It actually depends, i tuck it in when im at work.. (uniform) you present yourself in a way and you need to look presentable. Other times i would just wear it out..