FWIW: It's good practice to grease every nut, every bolt, every threaded everything, every time.
Stainless fasteners are a bit pricier than not, but at a $ or 2, at a time it's a managable expense when it comes to maintenance time. I'd put them in the "luxury" category of bike accessories.
I wouldn't care if surface rust developes. I'd care greatly if the fastener didn't work as intended because of rust.