We actually have a very good idea how much damage x-rays and millimeter waves do to the human body, since we've been dealing with both technologically for many years.
Because x-rays are used in medical devices that have to be approved by the FDA, we've developed some very good ideas about what sort of doses are harmful and which aren't. The damage done by the x-ray scans used by the TSA do indeed produce damage, measured in Sieverts - a scale used by medical technicians, nuclear power plant operators, NASA, etc. Generally a dose of 1 Sievert all at once is enough to make you sick. Radiation expose is usually measured in milliSieverts and a person who doesn't do anything dangerous will usually receive a couple of milliSieverts a year just from the sun and the radioactive decay of things like the carbon in their body. Going through a TSA x-ray scanner will give you about a microSievert (or 1/1000 of a milliSievert) unless you do something like stop in the middle of the scanner and daydream. Because flying on a plane involves going higher with less atmospher to protect you from radiation from outer space, even short flights tend to cause you to rack up 10 microSieverts and longer flights will give you more. So even if you fly every day the excess dose you're getting from the machines is ignorable, and if you are flying every day its only increasing the amount of radiation you're getting by 10%.
As to millimeter waves, well, I'm not sure quite what mechanism they could possibly use to be dangerous, since they're less energetic than the visible light that we're exposed to every day. If you're inclined to worry about stuff like that, I'd recommend worrying more about fluorescent lights (where all the energy is concentrated in a few frequencies and which we're exposed to for long periods of time) or cell phones and wireless access points (which have about the same energy as the TSA devices, and which we're exposed to for long periods of time).
