My understanding is that the Hanover accent is regarded as the most "correct", and my German colleagues tell me that the Saxon (Sachsen) accent is the least pleasant on the ear. That said, I would doubt very much whether it would make any difference for a beginner's course. Regardless of local accent and dialect, I would expect a course advertising itself as Hochdeutsch would take care to teach you the "proper" form regardless of where it is.