Just wanted to give my thoughts about this...
If you are posting a Fanfic of an author from FF.NET or anywhere else, I believe even if you are giving them credit, you should always ask for their consent first. With their consent AND constant reminders that it is THEIR story, I think only then its okay. As an author myself, I personally believe that there's nothing worse than being discredited or being denounced of your own story. It is incredibly rude...but other than that, I have no problem if you do post fanfics worth reading by others, all I'm saying is to ALWAYS be conscious of the actual author.