I keep running into what seems like a common need and wonder if someone knows how to do this. 

When you make a vertex object and then duplicate with symetry, it is good to have all of the soon to be welded vertexes perfectly aligned on the plane that will bring the two halfs of the object together.  Rather than go through the hundreds of vertexes and make sure they are all at say y=0, you should be able to select them all and make all of them have y=0.  Of course, you would not want to change the x or z values when you do this.  Then when you go to weld the two objects together, the welding process won't cause the new vertex to move to some value other than 0.   

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.

Attached Link: Align vertex movie (3.7M)

Great tip.  I saw the "move to drawing plane" command but never thought to use it this way.  It works great.  I use the duplicate with symetry command all the time but must admit I have yet to figure out what drives the axis it duplicates around.  Seems confusing to me.  Sometimes it works just like I expected and other times I end up with the duplicate on the wrong axis. 

Thanks