I always half tuck. Full tuck looks old lady to me (tho I am 39, haha, but I do wear decent styles and am slim, so it's easier). Untucked makes me look boxy and wide, unfortunately. And very fitted shirts show too many bumps.
I like the half-tuck, but only do it occasionally... Agree that the full tuck can read old-lady pretty quick, and even on the young and slim, can get a little bunchy and unflattering.
Though also agree with the PP that much of the reason for all the "tucks" is that all the tops are so darn boxy, drapey, wide, etc. these days. Could we have some fitted tops again, fashion-setters?