When a stereo pair is formed from a decoded view and a synthesized view, it is unclear how the overall quality of the stereo pair should be assessed through objective quality metrics. In this paper, this problem is addressed considering a 3D video represented in the format of multiview video plus depth. The performance of different PSNR-based metrics are analyzed in terms of correlation with subjective perception of video quality. A set of subjective data collected through formal subjective evaluation tests is used as benchmark. Results show that, among the considered metrics, the PSNR of the decoded view has the highest correlation with the perceived quality while the PSNR of the synthesized view has the lowest correlation.