That's a valid point. I didn't give that a thought. I've used stainless bolts on my other classic in the same situation and have had no issues in the last 5 years. I'd say that if you were looking at a single bolt of that size (5/16 or M8) then maybe but with 4 bolts sharing the 'load' as they say, I think it will be OK.
Incicidently, the crossmember and rear suspension arm bolts that Wins sell are stainless (which I've fitted), and am sure have been fitted to multiple cars out there. The rear long bolt the wheel end is a high tensile bolt which does have an incresed load on it though.