When I bought my first Mercedes I was told that once a MB was properly aligned, it stayed aligned. That's been my experience in more than 20 years of MB ownership.
The key is to have it done right the first time.
My 190D was delivered out of alignment. I took it to an independent who didn't get the job done. I took it to a Mercedes dealer and didn't have to touch it for the next 18 years of ownership.
My '91 300E hasn't been touched since I took delivery.