Priority lines at BOS funnel to 2 lines, one with full-body scanner and one with traditional. However, they feed everyone through full-body from both lines except when it backs up.
I always refuse full-body ("opt out") and instead endure a fairly thorough groping. My reason isn't about privacy or radiation, but rather that going through the scanner requires that you can't see your property on the belt at all times - in fact there is an uncomfortably long time where it is out of sight. Not safe given the proclivity of things walking away from there.