If I wear a long sleeve oxford shirt (Which I do most days at the office), then I wear an undershirt (Tee shirt actually) it's stretchy and fitted. If I wear a polo shirt or a tee shirt I don't wear an undershirt.
I just grew up wearing tee shirts under dress/casual, button up, collard shirts. I guess it's kind of like wearing a watch or a belt. Some people don't wear them. I do.
