Not entirely true. Stainless steal can rust. And one of the places it likes to rust is the threading of two metal parts. If it was my atty, I'd check the screws and post holes just to be safe.and thats stainless.. stainless doesn't rust.
Now, I have had juices do this if left in a wetted\juiced up atty for a week or two... so that's most likely the issue. But if your seeing the color creeping from the posts down the cotton while in normal use, I'd looking into things just to be safe. Or change juices and coil to see if it still happens. If I took the whole atty apart and inspected it and didn't see any signs of rust, I wouldn't worry about it.