I use square pots for veg and round ones for flower. I can fit more square ones under my veg light than round ones. I tried square pots for flowering but my spaces are small and I found I can better position the plants so that nothing is shaded with round pots. Square ones only give you 4 options.
Square pots are actually better for the roots. With a round one, especially one that is smooth sided, the roots will swirl around the inside of the pot. With a square one the roots get redirected when they hit the corners. Makes for a better root system. The round ones I use in flower have indentations that help redirect the roots and prevent swirling.