I have a confession to make. I've polished dies.
Polishing the expander definitely helps to make it pass through the neck smoother. There's a benefit, albeit small, to reduce the chance of pulling the neck and inducing runout or altering the shoulder/headspace dimensions.
I've polished resizing dies and it does make resizing smoother but that's it. In a practical sense it has no effect on anything. I chucked up a rod in a drill and used 000 steel wool with some oil to remove any fine burrs and then followed that up with some metal polishing paste. You don't want to alter the dimensions or remove the honing marks. The honing marks hold lube.
I am a loony.
