My vote is once a year if you drive less than 15,000 miles. That way you have the opportunity to take a look under the valve cover at other things, like the timing chain and the ports where the hoses enter the valve cover. I agree that it is good to keep a record of your work, and that you don't need to adjust a valve that does measure OK. But you do need to measure it.

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daBenz - 1970 220D