That's the assumption that people make that just because they're expensive, they have superior quality. QC is typically worse on their cheaper line of cars, but even S class cars have their share of problems.
As for me, I take both of my E-350's to Goodyear. They sell a 3 year alignment warranty. They have no issues doing the the alignment and it's typically $179 or something like that for the 3 year warranty. Potholes here mean I get it done every 6 months to a 1. All it takes is one hit and you know that it's off because the steering wheel doesn't line up straight anymore. There's other places that can also do the alignment but I hear bad things about the local Firestone stores but they might be fine somewhere else. The good thing with Goodyear is that they don't even bother with an upsell because they just assume I take it to the dealer. Typically on Mercedes, the front inner tires tends to wear out. Some claim that's because MB's alignment specs is geared for the Autobahn and at slower US speeds, the inners will wear out quicker. Should probably just rotate the tires more frequently than once a year during service.
