These are two “minor” bugs that the new WPF-based commandbars of Visual Studio 2010 are going to have since won’t be fixed in this release:
VSIP: VS 2010 Dec LCTP: toolbar with CommandBar.RowIndex = -1 not created in new row
VSIP: VS 2010 Dec LCTP: CommandBar.RowIndex of toolbar not updated after creating and making it visible
If your add-in uses more than one toolbar, it can affect you. I found a workaround using big values for the RowIndex property rather than -1 to ensure that the toolbars appear below existing toolbars.