Yes you will need to edit the one that comes with smartImager as there are a few items that are necessary in the unattend file.

Also, if you edit it via the Windows Image Manager, you may get an error on the <ComputerName> tag.  smartImager requires this tag to be %ASSETTAGREPLACE% and Windows Image Manager may balk at that when you go to save it - so you can just change it to any allowable string value, and then change it back to %ASSETTAGREPLACE% afterwards using notepad or any other text editor.