Rarely should you tuck in a polo, never a T-shirt, and tucking in an oxford again depends on the mood of your outfit. An untucked shirt denotes casualness.
For awhile I was wearing an untucked shirt with a tie, but I had the top button of my shirt done up, which really juxtaposed the whole outfit in a negative way. If you're going to wear a tie with the top button done up, all appropriate at whatnot, then you need to tuck in your shirt. It looks better.
And now for something completely different, and not really dapper as hell;