I would start with DocumentOpen and NewDocument. An additional level of complexity exists if you need to support ProtectedView documents; Word triggers a different event. I've found that if I try to check for that event (and it doesn't occur) it raises an error. I didn't had much luck and eventually it wasn't worth the time I was spending. I've posted an example below of some code that opens the Style Pane when a document is opened or a new one created (assuming the add-in is loading) and expands the style margin in draft view if not already expanded.
In my UI module:
Dim X As New clsAppEvent 'This is in the declarations
Public Sub OnRibbonLoad(objRibbon As IRibbonUI)
Do While Documents.Count = 0
DoEvents
Loop ' I find this useful as sometimes it seems my ribbon loads before the document.
Call Register_Event_Handler
' Other stuff
End Sub
Private Sub Register_Event_Handler()
Set X.App = Word.Application
End Sub
Then, in a class module I call clsAppEvent:
Option Explicit
Public WithEvents App As Word.Application
Private Sub App_DocumentOpen(ByVal Doc As Document)
App.TaskPanes(wdTaskPaneFormatting).visible = True
End Sub
Private Sub App_NewDocument(ByVal Doc As Document)
App.TaskPanes(wdTaskPaneFormatting).visible = True
End Sub
Private Sub App_WindowActivate(ByVal Doc As Document, ByVal Wn As Window)
If Wn.StyleAreaWidth <= 0 Then
Wn.StyleAreaWidth = 60
End If
End Sub
Other than the caveats I've noted above, the one problem I've had is if a user has Auto code in their Normal template as well. This has only come up once so I haven't investigated it.
I wish I could find the site where I learned about this (and from which the Register_Event_Handler was derived. If I find it I'll add a comment.