Here is a pair of auto macros that will do this if your really need it.
Code:
Sub AutoOpen()
'
' TrackChangesOn Macro
'
ActiveDocument.TrackRevisions = True
End Sub
Sub AutoNew()
AutoOpen
End Sub
These would go in your normal template. The first applies to every document you open. The second applies to every document you create from a template.
I do not think this is a particularly good idea because most people would not like having track changes on in every document. If you need help on what to do with the code, see...
Installing Macros
I've put a link to this thread in the troubleshooting portion of my page on
Track Changes.