I think the safest recommendation is to adjust the valves whenever you get a new-to-you vehicle that requires this procedure, and from that point on follow the 15k miles maintenance interval. This will keep the valves and their seats in optimal condition - that's why MB specifies the maintenance cycle.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles