Surface mounted Non-metallic Cable should follow closely to the surface on which it is laid. Code requires staples (or other fasteners) every 54 inches and within 12 inches of a metal box and 8 inches of a plastic one. The cable should either be run through bored holes or protected by furring strips within 6 feet of the access to the attic.
As far as running adjacent or perpendicular to ductwork, as long as the cable doesn't penetrate the duct, then there shouldn't be a problem with code.
