I tend to wear casual shirts (especially with jeans) untucked. Dress shirts are almost always tucked, at least for me.
i say tuck it in, i don't buy shirts that look better left untucked
I think it also depends on your body type. If you are athletic or muscular then you could get away with either. However if overweight or chubby then leave it tucked out. Also if the shirt is a slim fitting tapered one then it should be left out. When wearing trousers tuck in if jeans leave out.