When all the hair is pulled back from the face, as shown here on actress Gwyneth Paltrow (who has a classic square face), the squareness is very apparent. If you have sharp, angular features, a severe ponytail or bun highlights them. If you like that, great. But this look is a typical "don't" for square faces. And yes, I know Gwyneth is beautiful and looks good in almost any hairstyle, but she is all angles here (and you can't see her dress).
See softer ponytail options in this gallery of classic ponytails.
More on face shapes: