If I have a bunch of tagless or inccompletely tagged tracks under "unassigned", I then take note of which they are, and then browse under the "File Location" section of the tree.
Then it's a fairly simple matter of highlighting a whole albnums worth of tracks, and setting the Artist / Albumname / Year etc for ALL the tracks for that album, and then updating the song titles (which you know from the file name hopefully).
I hardly ever have this problem with MJ, but as I said earlier, it seems to happen frequently when I'm ripping on a second PC and storing the files across LAN to the server in my room.
I seem to be able to get around this wierdness by just saving the second PC's files locally, then copying them manually to my server, and then re-importing...
Hope this helps ya!