I always opt for the manual pat-down, just in case. Who knows how well the machine is configured to blast me with the right amount of x-rays and not too much. Who knows how carefully it was transported to the airport, assembled at the airport, tested at the airport, etc.
Half the time, everybody there looks at me like I'm wearing a tinfoil hat when I politely request the manual pat-down.
If you tell them you can't lift your arms over your head they're required to just wave you through without a pat down. Or at least this was the case until 2020 or so when I stopped doing this because I would get routinely threatened by TSA employees who were frustrated at how much this exception highlighted what a joke their jobs were.
Half the time, everybody there looks at me like I'm wearing a tinfoil hat when I politely request the manual pat-down.
The other half the time nobody bats an eye.
Depends on the airport and culture, I guess.