I'm not a scientist but I think that rust could in fact affect the integrity of the shock and therefore its performance. My truck sat on the lot for about 10 months, I rarely park it indoors now, and Cleveland winters are brutal. I would not use the word light when describing the rust on the bottom ends of mine.
The rust on my shocks is almost as pathetic as the rust on my frame (which I get is purely cosmetic) but at least the shocks are a replaceable part.
The ranchos are cheaply made. This can be seen in the overall appearance and how quickly they rust. I will be upgrading one day but trying to get a decent amount of mileage out of them before they rust through.
I was worried about this when I tuned my Colorado. Originally went for the 89 then went and changed it to 93. I was sweating for a couple months that I deleted my factory tune by not restoring before going to the 93. Especially when the tuner says something along the lines of copying previous tune. Long story short tho, when it came time to get rid of the truck I restored it to factory with no issues.