From all the comments I've heard, the lower 2 rails do not fail traumatically, and require removing both heads to get the front timing cover off.

If you're already pulling everything else off the front of the engine, and both heads, you might as well change the lower guide rails.
On the other hand, if you only have the one head coming off, and are not already removing everything else on teh front end, I'd leave them be.

I just got done refreshing a 500SEC engine, out of the car, and didn't replace them, if that's any help in the decision.

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-Josh
Testing the cheap Mercedes axiom, one bolt at a time...