I see two definitions of karma applicable here:
The first is, imo, as woo as believing in god or homoeopathy. The second is intuitively true.
how many atheists out there... ...dictate their lives with a consideration toward the idea of karma?
I don't consider it to be karma, but I generally try to be good to people. Not nice, necessarily, but fair in my dealings with them, and I expect the same in return.