Here's what I do: I drag the object until the seam is as close as possible to the Y plane (I realize I could also select several verts then move the plane there, but I like this way). Then make a marquee around the verts I believe need to be welded. Perform "move to drawing plane." Now I have all of them absolutely flat along the Y-plane, and I am lined up on it; I can duplicate with symmetry and weld with a very small setting. One thing I haven't figured out a neat way to do is when pre-mapping. I've been doing above, dragging the map for the "left" side into a corner, duplicate with symmetry, drag the map for that into another corner, weld the two halves, then go into the UVmapper and flip one map and join the split verts. Not really neat.