This is a pretty simplistic nuglet, but I went about my business for two days living with a non-centered navigation menu until this simple fix presented itself. The issue is one most of us encounter: Anonymous users see 4 menu items in their Navigation List, Registered Users see 5. This is usually not an issue unless we need to center it on the page.
CSS-wise we use an Outer Div at 100% width, then place the Navigation List in an Inner Div of a set width using auto margins. The problem is specifying the width of the inner Div because we don't have a set number of menu items.
The solution is adding the Inner Div based on role, like so.
<CSControl:ConditionalContent ID="ConditionalContent1" runat="server">
<CSControl:NavigationList ID="NavigationList1" runat="server" Level="0"