This is simply a personal or office level choice. If the documents are shared by more that one user, they probably should be stored on a network server. If the documents are personal to the user, and will likely never be used by others, then storing them locally certainly makes sense.

   If document assembly speed is an issue, and the network server is in a remote location that is accessed over the Internet (as opposed to a local area network), you may wish to consider 'synchronizing' the network files to the local harddrive on a regular basis. Pathagoras has tools for this. See synching.