Never really gave it much thought. But I had two square tables, a breakfast table and a dining room (formal) table. Mother had a round breakfast table. Honestly, it would just depend on what set I happened to like the looks of and whatever shape it is, that's the shape I'd accept. (Guess you can tell what level of decorating and design talent I possess, right?)
Sounds simplistic, but a round table you can squeeze extra folks in and not so much at a square table.