Well, at the risk of breaking the conformity that has so far been posted, I would have to say that I do believe in that said idiom.
(I do believe that's considered an idiom... Correct me if I'm wrong, please.)
I see the universe in a logical and yet philisophical manner. When one does nothing but evil, nothing but evil shall happen upon them. If one does nothing but good, nothing but good shall happen upon them. That is the basis behind karma, and I believe in it completely.
Does it actually occur? Well, that depends on your perspective. If you see a child throwing sand at another child and then getting reprimanded by the parent. One could see this as karma in effect. However, one could also see this child getting reprimanded as a good thing because it teaches them a life-lesson. I would love for karma to work infallibly, but this is not the case.
I suppose a better philosophy that I believe in would be, "Everything happens for a reason."
It really does. Every action you take occurs for a reason. Even if you just sit on your couch doing nothing, that will lead to you either missing out on something good or possibly avoiding a harmful fate. The point is, every thing one does occurs because of something and causes another. This process compiles together to make what is now so popularly referred to as "The Butterfly Effect."
Sorry if my post doesn't sound too logical. For some odd reason or another I can't very acurrately articulate myself this evening. :/