The tips I gave are to ensure that all the vert will snap to the yz plane going through the origin. If you are not in world mode and have retain component spacing unchecked you will not be gauranteed to have all the verts aligned to the origin.
See Jay's post. If the verts are not aligned to the origin there will be a space between the mirrored halves.
"If I have seen further it is by standing on the shoulders of giants." Sir Isaac Newton, 1675