Most existing studies on pinching-antenna systems optimize the antenna positions or activation states under a prescribed waveguide deployment, where mutually parallel waveguides are commonly adopted. However, it remains unclear whether, and under what conditions, such a deployment is optimal from a communication-performance perspective. To fill this gap, this paper investigates long-term waveguide deployment optimization for pinching-antenna systems over a rectangular service region. We first consider the fundamental two-waveguide case and derive a closed-form sufficient condition, jointly determined by the waveguide height and the service-region aspect ratio, under which a deployment parallel to the longer side of the service region is globally optimal. We then consider the general multi-waveguide case, where the orientation and offset of each waveguide are jointly optimized without prescribing any deployment pattern, and develop an adaptive exchange algorithm for the resulting nonconvex optimization problem. We further show that, within the parallel class, the waveguides should be aligned with the longer side of the service region, while in the large-height regime, the globally optimal unrestricted deployment asymptotically concentrates all waveguides on the centerline along the longer side. Numerical results validate the proposed algorithm and reveal how the preferred deployment geometry depends on the waveguide height and service-region shape. In particular, nonparallel deployments can provide noticeable gains in intermediate-height regimes, whereas parallel or center-concentrated deployments become favorable in other regimes.

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