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">
<ContentConditions>
<CSControl:UserPropertyValueComparison
ID="UserPropertyValueComparison1" runat="server"
UseAccessingUser="true" ComparisonProperty="IsAnonymous"
Operator="IsSetOrTrue" />
</ContentConditions>
<TrueContentTemplate>
<div class="InnerAnonymous">
</TrueContentTemplate>
<FalseContentTemplate>
<div class="InnerRegistered">
</FalseContentTemplate>
</CSControl:ConditionalContent>
<CSControl:NavigationList ID="NavigationList1" runat="server" Level="0"
Theme="drive" Property="navigationItems"
RenderChildrenInContextMenu="false">
....