AFAIK, there is no switch (and I believe the default actually varies from
version to version!)
However, it's pretty straight-forward to use code to change it:
Sub SetZeroLength()
Dim dbCurr As DAO.Database
Dim tdfCurr As DAO.TableDef
Dim fldCurr As DAO.Field
Set dbCurr = CurrentDb()
For Each tdfCurr In dbCurr.TableDefs
For Each fldCurr In tdfCurr.Fields
If fldCurr.Type = dbText Or _
fldCurr.Type = dbMemo Then
fldCurr.AllowZeroLength = True
End If
Next fldCurr
Next tdfCurr
End Sub

Signature
Doug Steele, Microsoft Access MVP
http://I.Am/DougSteele
(no e-mails, please!)
>I would like to be able to set the default value for Allow Zero Length to
> True.
[quoted text clipped - 7 lines]
>
> Any help you can give will be greatly appreciated - BCP.