I'm struggling with the same issue. On other sites I have simply centered the cell that the nav include is in but on the site I am working on now this does not work.
I've even tried placing it in a div and centering the text with
text-align: center!important;
but it still is left justified.
Forcing left justify produces awkward results when the page names are of varying lengths. Centering the entire menu and the text on each button gives a more balanced appearance.