I always loosen the strings off - not because of any other reason than I've seen instances where the guitars have been thrown around so much that if they land face-down, the strings actual press into the frets or fretboard. Granted, not really on bass but have seen it 4-5 times on guitars I've bought, so I err on the side of caution and (touch wood) have not had any problems.
Cheers,
James