If you don't include any ElementID lines related to the mainbar, then it will go back to its default position (Mapping, for skins, doesn't count, they will not affect the position), however if any sub-elements of the mainbar are moved, you do have to include the actual mainbar line and its coordinates. If you're not doing anything too funky, pretty much the centering coordinates are:
X=(Screen width - bar width)/2
Y=Screen height - bar height (not too sure about Y)
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