I use SS wood screws A LOT, and to be honest, they've been just fine. We live across the street from an ocean beach; salt spray, constant rain, fog, you name it, but I haven't had any issues with the stainless steel screws.
Having said that, I buy them by the 100 count box. I hate the quality of the junk in the blister paks.
I get my stuff from a local shop, Gibsons Fasteners, who cater to the marine industries up here. Not saying the products are the ultimate in quality but I think they'd be concerned enough to make sure the clients were getting decent product.