According to Final Fantasy XIII-2 producer Yoshinori Kitase and director Motomu Toriyama, you may want to hang on to your game clear save file from Final Fantasy XIII. Why? Because it will be used to in the sequel to some extent. What exactly it will be used for is unknown, as the super duo didn’t go into specifics.
It’s likely there will be some special items or, even better, cut scenes, that can only be accessed if you have a save file from the first game. I’m interested to see where this goes. As those who have finished the story in Final Fantasy XIII know, it’s pretty easy to keep a post-game save file, since much of the game’s side elements are designed to be finished after the main story.