It seems to me that the strategy of having the toolbar created by the add-in
during by Auto_Open is not compatible with persistence of any toolbar
properties. If the toolbar is created each time PowerPoint starts, it will
always have the same properties (either those set explicitly or the default
values, but never whatever the most recent settings were), right?

Is there another way to create a toolbar that is more persistent? I have
tried removing the Auto_Close entirely, but even then the toolbar is
re-created each time PowerPoint starts (despite my having included an Exit
Sub for an error indicating that the toolbar already exists). I have tried
NOT setting the Visibility property, but then the toolbar is always invisible
when PowerPoint starts.

Here's the add-in code I'm using (relevant portions):

Static Sub Auto_Open()
   Dim oToolbar As CommandBar
   Dim oButton As CommandBarButton
   Dim strMyToolbar As String

   ' Give the toolbar a name
   strMyToolbar = "MyToolbar"

   On Error Resume Next

   Set oToolbar = CommandBars.Add(Name:=strMyToolbar, _
       Position:=msoBarFloating, Temporary:=True)
   If Err.Number <> 0 Then
           MsgBox "Error number and description = " & vbCrLf _
           & Err.Number & ": " & Err.Description
         Exit Sub
   End If

   On Error GoTo ErrorHandler

   Set oButton = oToolbar.Controls.Add(Type:=msoControlButton)
   With oButton
        .TooltipText = "Set Document Properties"
        .Caption = "Set Doc Props"
        .OnAction = "SetDocProps"
        .Style = msoButtonIcon
        .FaceId = 487 'info icon
   End With

   Set oButton = oToolbar.Controls.Add(Type:=msoControlButton)
   With oButton
        .TooltipText = "Unload the Add-In"
        '.TooltipText = "Unload the " & strMyToolbar
        .Caption = "Unload Add-In"
        .OnAction = "UnloadAddin"
        .BeginGroup = True
        .Style = msoButtonIcon
        .FaceId = 1640 'exit door icon
   End With

   oToolbar.Top = 150
   oToolbar.Left = 150
   oToolbar.Visible = True
   
NormalExit:
   Exit Sub

ErrorHandler:
    MsgBox Err.Number & vbCrLf & Err.Description
    Resume NormalExit:
   
End Sub

Sub Auto_Close()
   
   Call DeleteToolbar

End Sub

Sub DeleteToolbar()
Dim oToolbar As CommandBar

Set oToolbar = Application.CommandBars("MyToolbar")

oToolbar.Visible = False
oToolbar.Delete
       
End Sub

Sub UnloadAddin()
Dim oAddin As Addin
Dim Msg, Style, Title, Response

Set oAddin = Application.Addins("MyToolbar")

Msg = "Are you sure you want to delete this toolbar " _
& vbCrLf & "and unload the add-in?" _
Style = vbOKCancel + vbQuestion + vbDefaultButton1
Title = "Unload Add-In?"

On Error Resume Next
   
Response = MsgBox(Msg, Style, Title)
If Response = vbOK Then
   'Call DeleteToolbar     'Moved to Auto_Close()
   oAddin.Loaded = msoFalse
   'Addins.Remove "MyToolbar"     'Decided only to unload, not remove
End If

End Sub