Thanks for the feedback on the 7 series.
I am at a bit of a loss. The radar sensors are below the front bumper, but there is another sensor, a "front-facing camera" in the windshield. And ACC windshields are different part numbers than windshields with just a rain sensor. Per Volkswagen:
"The Touareg has a forward-facing camera at the top center of its windshield, four ultrasonic sensors on its front bumper and a pair of radar sensors near the front foglights. When ACC is on, these sensors monitor the traffic in front of the Touareg and adjust its speed accordingly. The following distance is determined by a setting you control. Move the Distance switch left or right to decrease or increase this distance."
Unfortunately, erwin does not cover this version of ACC, only the ACC in models up to 2010. It does include the statement that manual calibration of the master radar sensor (the right one I believe) is required any time the module is replaced, there are any adjustments made to the front or rear suspension, or there is any impact to the front or replacement of components up front like the bumper.
I believe the alignment is precautionary to make sure that the car is not dog-legging. Erwin does state that if the radar sensor is 1 degree out of alignment, the focal point will be 7' off at maximum range. My only data point here is that I aligned the car at an indy shop 6 months ago due to tire cupping, did not recalibrate anything, and saw neither an improvement nor deterioration in any of the ACC functions.
The dealer service advisor also stated the the ACC units go out of calibration over time and should be recalibrated (again for $550) every two years. I'm mulling that one over in my head; I'm a little skeptical as I cannot figure out what the mechanism for this is, but I am keeping an open mind.