Is it a good idea to put spin on projectiles? I know it wouldn't make much difference with stones or BBs, but what about arrows, etc? This would surely increase accuracy.
Well, there's spin, and then there's spin ... it all depends on the direction relative to the projectile's motion. Ideally, the spin should be transverse (both laterally and vertically) to the line of forward motion, in order for it's gyroscopic effect to be most beneficial to maintaining it's flight path. The greater the distance travelled, and the greater the spin, the more effect the spin will have ... which is why the difference between gun barrels that are rifled, rather than smooth bore, is very pronounced. Conversely, the shorter the distance, and less the spin, the less effect it will have.
On arrows, particularly for medium and longer range shooting (but not for short range), spin is helpful - which is why some people pay extra for "helical", rather than straight, fletchings. I have helical fletchings on the arrows for my longbow.
Here's a diagram I found for you: