I moved the vertices in the original geometry file. You raised an interesting question though - does morphing or magnetising affect the weld differently? That would depend on the order in which the calcualtions are done. As far as I can recall from experience without devising specific tests, once vertices are welded they stay welded.

I did have a strange experience with joints splitting apart under some circumstances even though the vertices were perfectly aligned, and I never got to the bottom of that except that it seems to have been a bug that was introduced in P6. It may well have been fixed in later Poser versions, I never repeated the test.

http://market.renderosity.com/mod/forumpro/showthread.php?thread_id=2301883

http://poserpros.daz3d.com/forums/viewtopic.php?t=46304

I think you may have to already be a member to read the PoserPros forums: they're frozen now, but still a useful source of information.