Well, there are a couple different methods that focus on C1 and C2.
I was never trained in it (beyond just a class so we knew about it...one would have to take extensive seminars outside of school to become proficient), but we have been seeing an Atlas Orthogonal chiro the last year. We've loved it, though I think they've watered down their practice to make people happy, b/c they will also adjust thoracics and lumbars. AO is supposed to focus on the top vertebra and let it help the rest of the vertebrae and the body. I'm a purist when it comes to that.
My school specialized in Toggle Recoil, which is generally done on C1 and C2. It's a very interesting technique that's not supposed to put much actual force into the spine (though when you're just watching it *looks* like it). I had the most powerful adjustment of my life at the school's clinic, from a student practicioner. It rocked my world, and set my head on straight. When I look at pictures of myself before, while going through college, my head was tilting further and further. Now that's cute look as a kid and even up into your early 20s. But if it had continued, the cuteness would have started getting lost! After that adjustment on my C1 vertebra, I bounced down the hallway (after adjustments the patients there must rest, then come back and get rechecked), having had my equilibrium changed drastically. Afterwards, my pictures showed my head nice and straight!
Though they are generally not strictly upper cervical practicioners, I don't have much problem with the other ways of adjusting the neck vertebrae. Gonstead technique is incredibly specific, Diversified can be used specifically, and so on. I personally had a toggle recoil focus on the upper cervical vertebrae, inside of Network Spinal Analysis which does look at the whole spine.
The risk of an adjustment is actually less than the risk involved in tilting your head back to have your hair shampooed at the salon. And that risk is ONLY if you had a preexisting condition going on with your arteries...
There is nowhere near enough force in ANY technique to actually break anything, and the meninges covering the spinal cord, protected by the vertebral canal in each vertebra, are insanely strong, and you have to put an incredible force into them to do anything bad to them.
And the benefits of having those upper cervical vertebrae in better alignment are immense and seemingly never-ending. It's astonishing, what friends and colleagues (and me, though my practice was short-lived, only 5 years for many reasons, some good and some stupid) have had change in their clients. And my extended family, who have all been into chiro even before I went to school for it, have had some pretty amazing things happen as well.