Stainless will still rust unfortunately, but should hold up longer than regular steel exhaust systems.
The main thing to consider is the grade of stainless used to manufacture your exhaust will make a difference. Some have higher nickel content which should make it more resistant.
But remember that heat will increase the chemical reactions that occur.
What i would suggest is to clean the underside of your vehicle periodically during the winters when raod salt is applied in your area.
