This is one of those things where there is the "gold standard" and then plenty of other methods that are still perfectly decent. Walnut shells will give you the best results. They sharpen & moisturize your pins to keep them lasting extra long.
Cotton batting will add a tiny bit of moisture, but also has the possibility of developing mildew inside if in an area of extra high humidity and not periodically washed/dried ... and it will dull rather than sharpen your pins. Poly batting/fiber fill adds no moisture and will dull pins very slightly more than cotton. But honestly, the difference is small & I've bought, made & used fiber fill pin cushions plenty of times. Honestly, my pins bend well before they ever rust or dull so it's not a factor for me. I use silk pins & they are not very strong so I end up replacing probably them every 2 years anyways.