I opt out of backscatter, but not MMW, for health reasons. I had radiation therapy a couple of years ago -- not for cancer, but the doctor in charge of the therapy was a radiation oncologist.

I asked him about the scanners afterwards, and he said to avoid the backscatter ones; the answer didn't give me any context on whether that was in general or given the dose I'd received, but I have always assumed the latter.

Either way, I figure that if anyone is going to be up on the safety of radiation, it'll be someone in his profession. I opt out. I'll volunteer outright that it's because I've received radiation therapy, and my doctor says to avoid them. Usually, that gets me redirected through the metal detector rather than a patdown.

At the same time, with fewer safety concerns about MMW: I have no particular privacy concerns although I respect those who do.