Perhaps having edges set to hard while subdividing minimizes this distortion in the UVs, I haven't tested that but it's easy enough to test outside of Poser. Generally if I UV something at low poly and subdivide, running a relax can clear up any distortions. Usually that's not necessary though. Sometimes a relax can cause additional problems. I would think that a sophisticated subdivision algorithm such as OpenSubdiv (what Poser uses) has systems in place to counteract UV distortions so it's puzzling.