There used to be an option to center the panel in the screen, but is has gone since the last time I used xfce. Can I center my panel using some option in the xml files for the panel?
You could center the items on the panel by placing expanding separators on either side of the items. This will push the items from both sides and place them in the centre.
You can also resize/move the panel and place it where you want. To resize/move it, right-click panel-> Panel -> Panel preferences, uncheck "Lock Panel", resize by changing "Length", and drag and drop where you want it.