In Q1 2016 SP we have introduced the ToolWindowCreated event which is raised when auto generated RadPaneGroup is created and can be helpful in this case as it provides an easy way to set TabStripPlacement to the new created RadPaneGrups:

private void RadDocking_PaneGroupCreated(object sender, ElementCreatedEventArgs e)
        {
            var paneGroup = e.CreatedElement as RadPaneGroup;
            paneGroup.TabStripPlacement = Dock.Top;
        }