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.