I just had the timing chain and upper rails replaced on my 420. I believe that the common rule of thought here is that the rails turn amber color from heat and age, become brittle and then break, resulting in major engine repairs.
From what I can glean, the rails are not that more expensive a part and would make sense to replace while the shop is in replacing the chain and tensioner.
The lower guide rails are more difficult and may not be replaced in some instances if not needed.
My rails, at 102000 miles were amber and one was cracked, so it definitely was due.