I buy all the fasteners for the engineering department that i work in and we have never had any problems with stainless steel fasteners and 99% of the fasteners we use are stainless. The only time we use anything else is for high tensile applications where we would use black high tensile bolts.

If i was replacing the fixings on my car i would use grade A2 stainless fasteners. Grade A4 is also available but they are a lot more expensive.

Might be worth others giving their experiences of using stainless fasteners on cars as my fasteners at work are only used on machines used in research laboratories (nice clean & dry environment). Maybe the stainless properties break down in certain applications on cars due to heat or vibration (but i doubt it).

There will be some places on a car where stainless will not be suitable due to the high torque settings required.