Generally speaking, there hasn't been definitive proof of rifled barrels positively affecting dart flight. The first major authority on the subject would be Ben Trettel, a.k.a. Doom from the NIC. This blog post explains why rifling hasn't been shown to help darts. Basically, darts are generally already stable in flight because they are weighted more toward the front, and the center of pressure sits behind the center of mass. At most, there have been only anecdotes in support of rifled barrels. Quite simply, it's easier to fix the dart's mass and drag issues than it is to achieve spin stabilization.
But what about larger ammo, or homemade ammo? Nerf arrows, Demolisher rockets, and Titan missiles all use spin stabilization. However, these are all very lightweight for the volume they take up. Since an increase in mass would decrease ranges by a significant margin, the extra drag from spin stabilization is an acceptable tradeoff for stable flight. Meanwhile, the foam backer rod used for stefans isn't always straight, especially for the larger sizes. This area will be the focus of my current experiments.