I've been to all three checkpoints at Dulles: east & west main checkpoints and the Diamond Lane very recently. The general set-up is a magnetometer right next to a Cancer Machine. The Diamond Lane has one of each. Here's my advice and an observation:

1. Avoid the Diamond Lane. You have only a 50/50 chance of getting selected for the Cancer Machine. If you can, I would stop across the hallway from the Diamond Lane and pretend you're on the phone inorder to be able to observe if the Cancer Machine is in operation. If it is, or if you're not sure, head for one of the two main checkpoints.

2. Main checkpoints: The west checkpoint has the Cancer Machines to the right of the mags. The opposite set-up exists at the east checkpoint. After clearing the ID checker, simply go into a line heading towards the magnetometer. Each scanner and mag line has its own carry-on x-ray. There is a chance that the moat dragon will divert you to the Cancer Machine, but, there are usually enough sheep lining up that it's not a problem. Your chances of getting through without being irradiated and/or groped are significantly less at the main checkpoints.

Observation: As another FTer has posted, I also noted that the lines for the magnetometers were 2-3 times longer than the lines for the Cancer Machines. There were no opt outs at the machines because people had already opted out by choosing the lines that they picked. Unfortunately, the people who got in line for the Cancer Machines appeared to be our foreign guests, who were probably unaware of what was about to happen to them.

If one conservatively assumes that the mag lines were only twice as long as the Cancer Machine lines, 2 out of 3 people opted out. When given the choice, humans don't normally pick the longest line. So, self-opting out can be the only reason for voluntarily getting in the longer of two lines.

You will NEVER see this observation in PV.