In over 20 years of defragging i've never lost a file yet. Well, at least not to the defragging process ;)

One thing to keep in mind ... if for whatever reason you need to shut down your computer or otherwise stop the defrag process before it's completed, use the "stop" function of the defrag software. This will make sure that any files that are in the process of being moved at that time are safely stored back on the drive. If you stop the process without letting defrag do the stopping then you risk losing whatever file contains the blocks that are currently being moved.