Up to the 5 series burners 4x was definitely the recommended speed. Since then 8x has become a common scanning speed, matching the Benqs.
When I got my Liteon 1635S there was a lot of firmware development being done here and I'm sure that we all scanned at 8x either in Liteons or Benqs.
There's suggestion that higher scanning speeds might test the media more than lower speeds.
I don't think that a scan in itself tells the whole picture which is why a TRT is often very useful.
I'm really only quoting others with more knowledge/experience than myself like DrageMester and C0deking.