[Greg McKaskle]

It is more than a convention, the wheel shape determines if the robot will be able to pivot.

Imagine drawing a circle on the floor, with diameter matching the diagonal of the robot drivetrain. Place the robot on the circle with all four wheels touching the circle. If you wish to pivot the robot so that the wheels follow the path of the circle, something had better provide a resistive force. One orientation will align the rollers tangential to the circle, providing resistance with the floor and allowing the pivot. The other orientation will align the rollers perpendicular to the circle. Practically no resistive force means practically no pivot.

I believe all other vectors are equivalent between the two.

Greg McKaskle