hardest thing on electrical system ?

If your car has a computer, don't wash the engine period.
But the older cars under consideration, I only do it when I am going to drive the car so the engine bay heats up and dries the connections out. (Leaf blowers help too)
I would think retained moisture is the problem.
Yes, clean metal might tend to corrode more than greasy metal. Who knows?
But I like clean.

PS. Keep the water off of the master cylinder.
Ever wonder why the brake fluid turns amber?
Look it up.