For subwoofers operating in the subbass frequency region there is nothing wrong with a perfectly square enclosure as the wavelengths are far too long compared to the size of the enclosure for it to matter.
About the only time it would be an issue would be for people doing crazy IB projects in their home where they are using attics, basements, etc as airspace........they might have issues with standing waves.
But for any enclosure that will fit into a car.....no.