I would leave them until they fail.
My friend W140 S600 is doing this job next week and was talking to one of the mechanics at the dealer about what they recomend if one snaps off. He asked them about the kit MB makes, the guy said it was $900 and they never use it. They pull the head and send them out, I keep hearing the $2k figure floating around but I'm sure if you can pull the head yourself it would be cheaper.

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2016 Corvette Stingray 2LT
1969 280SE
2023 Ram 1500
2007 Tiara 3200