I'd say light corrosion on the steel strings in the tenor / treble won't affect the sound noticeably. I guess corrosion is more of a potential concern to a purchaser - does the amount of corrosion relative to the piano's age indicate the piano has been kept in a non-ideal environment? Will the corrosion cause any strings to break during tuning or hard use?
I've seen this many times on acoustic pianos, especially grand pianos, where you can get a really good look at the strings, as opposed to an upright, where the strings are more hidden behind the front panel and action. And, I've seen it on some later model pianos, as well as older pianos.
Of course, I'm talking about light rust/corrosion spots, and not heavy corrosion. I have seen pianos with substantial corrosion on the steel strings, to the point where I would not want that piano. The lighter rust spots can be cleaned/buffed off with some 0000 steel wool, but then the loose fibers of the steel wool would also need to be vacuumed/dusted off the area beneath the strings.
I've also seen older pianos where the strings were tarnished from age, which is normal. But there is a difference between natural tarnish and rust spots.
I agree with Ben, in that it probably doesn't affect the tone enough to be noticeable, but I have no scientific evidence of that.
Just my .02, rust free...