to be honest what I do is add the tag "new" to the keywords tag. I know, sounds stupid as you can just make a playlist that sorts by the date added. However, I do this because then I can un-new it later after I've listened to it.
Ideally this Additional Tagging plugin (forget the exact name) would be best, as it can add tags automatically but as far as I understand it - it cannot just add the tag to the files in the inbox but will add the tag to any file that has been modified, which I'm sure there is a good use for this but it escapes me why anyone would want to add a tag every time any mp3 file is modified.