My mechanic said that the tensioner rails should be replaced every 100K. He said that there is no way to tell when they are getting old and brittle (since they are plastic). If you purchased a used vehicle and there is no records that they have been replaced then you are better off to have them replaced to save you the $$ that it is gonna cost you to have it fixed along with the valve job it will need.
Guess the ol' saying of "an ounce of prevention...." is true (at least in the case of MB maintnenaince).
: