Parents and Teachers are often concerned with their kiddos' self-stim (self-stimulatory or stereotypical) behavior. They ask me if it is okay to allow the kiddos to stim or to stop the stimming. There is some contradicting information out their regarding stereotypical behaviors. I have heard some researchers believe that to interfere with a child's stereotypical behavior is to deprive that child's brain of necessary input and can even result in brain damage. I've also met professionals who believe that one must "get down to the child's level" by engaging in the same stereotypical behavior in order to establish a relationship with the child. What I recommend to parents and teachers alike is to comment on the activity the child is engaged in and then redirect the child to a more appropriate behavior or activity.
For example, many of my little ones enjoy turning toy cars upside-down and spinning the wheels. We comment on the activity: "You are spinning the wheel" or "Watch the wheel go round and round" and then show the child (model) how to more "appropriately" play with the car by rolling it on the ground. If it is a bit more difficult to find an appropriate counterpart to the stereotypical behavior such as flicking the lights on and off or repeatedly flapping a hand in front of the face, we comment on the behavior (label the action) and then redirect the child in something completely different to engage him so he cannot continue the behavior. Try it!
However, sometimes it is important to have something highly rewarding like when you are teaching the child something new. Allowing the child to engage in the stereotypical behavior is often more reinforcing than food rewards.