It's clear that you can opt out of AIT (mm wave) scanning if you don't have SSSS and uncertain otherwise.

From the TSA website, https://www.tsa.gov/news/press/factsheets/technology , "Most passengers have the opportunity to decline AIT screening in favor of physical screening. However, some passengers will not be able to opt out of AIT screening if their boarding pass indicates that they have been selected for enhanced screening."