I haven't tried doing this in quite some time (iTunes 7 era), but I seem to remember simply editing the XML file iTunes creates was NOT sufficient in updating the Date Added field in iTunes.
In case you're curious why I would want to edit Date Added, it is because I most often enjoy listening to music with iTunes sorted on Date Added field. Most of the time this is a logical order for me, but occasionally (when importing more than one album at a time) iTunes imports a batch of songs in a seemingly random order, so I might have very differing albums intermixed. In these cases, I'd like to go in and make sure the Date Added times are grouped by album.