Stimming, like all autism topics, is a matter of some controversy. Some see it as a negative behavior to be eliminated, others see a natural form of expression for an autistic person, others a relief from environmental pressures and so on. I see no harm in stimming and Conor enjoys it so I don't try to prevent it. To the contrary when we visit a store or mall I look for a straw, his favorite stim object, for Conor to enjoy.
Conor stimming in the nearby volleyball pit. Any time we walk by Conor goes over to stim (sifting sand). He will usually sift the sand for as long as I let him. Stimming by sifting sand, for what seemed like endless periods, was one of the first indicators to us that there was something "different" about Conor and helped lead us to seek a medical assessment.
Conor enjoying some treats, and stimming with a straw, during a break while seeing Shrek 3 at the theater.
Conor multi-tasking, reading a Dr. Seuss book, while stimming with a straw with his other hand.
Conor sitting on the side step and stimming with a straw, while waiting patiently for Dad.