I've actually never heard anyone say that planets rotate in a CCW orbit around the sun. Because of precisely the reason Slick states.
However, if such a thing were said, then I'd imagine the astronomers have decided on some agreed-upon, fixed point of reference. One might venture that such an agreed-upon point of reference might have to do with the sun's magnetic field and magnetic poles, but those reverse every 11 years, so then again, probably not. Anyway, if we agree upon a fixed point of reference in space, even an arbitrary one, then we can define the direction of the orbits of the planet.
Or maybe not.