The answer is..
Well, I have watched this thread morph with a fascinated interest. What was originally my question about when to change the timing chain morphed into a fascinating discussion about which direction the link should face. Love the detail.
Well anyway, I finally got the answer I was looking for from the Technical Letters section of the most recent Star magazine.
The editor says (for duplex only):
100k miles: change not necessary, but will noticeably improve performance.
150k miles: change recommended soon.
200k miles: change now, performance will improve greatly.
If you have a single chain, then convert it to duplex as soon as possible.
Thanks all for your comments. :p