In a discussion at the parts counter at a Benz dealer I was told that the proper procedure (at least for my car, a w124) at a dealership was to strap down the car to simulate the weight of a driver (and passenger/s?). Since then I've read similar claims several times in various tech articles and forums.

As you mention, "... alignments should have proper weighting ...", the way I understand they achieve this is by strapping the car and winching it down to simulate the weight.
Perhaps I have it wrong though, is there a another procedure for simulating the weight? Have someone sit in the car, maybe use bags of sand or something?