Great question!
The ones that sometimes come in Stern's have been known to be rusty.

Basically they need to be as shiny as possible, with a clear and even gloss. Don't get cheapo ones. Rusty or scratched balls...well, think about a ball of sandpaper rolling around on your nice playfield!

The pinball life balls are great...oh so shiny!

People have their own preferences, but those are my go-to. Also be sure to inspect and swap them out periodically. Like oil in your car. They wear and break down.

There is no real rule as to when to replace them aside from "when they need it"

Pulling out a ball that's still in fantastic shape won't accomplish anything.