As has been said, you shouldn't pressure wash sidewalls as it can penetrate the rubber and damage the sidewall. I don't know if this is related to steel reinforcement, but many years ago we had an MG club member who's tyres started making a funny noise. It turned out that the rubber had split, allowing saltwater into the steel wires which had rusted and were breaking up. This is the only occasion I've ever heard of this happening.
If you pressure wash wires, I think the biggest danger is forcing water into the spoke threads, and also into the hubs. If it can't dry out it will cause corrosion. Although maybe not that much on stainless.
