I’ve washed used strings by putting them in my jeans pockets when I do laundry. They come out clean and play ok. I’ve also washed a lot of new strings this way by accident and it doesn’t hurt them.
I’ve seen a yoyo string break and the yoyo went flying across the room. Nothing was broken but caused a flat spot on my throw. They are cheap enough to change, cheaper than a.good yoyo, window or 60" flat screen. You can wash them if its simply dirty from a spill or dirty hands but hang dry. Just my 2 cents.
This is from the description of the Toxic’s Markmont Dragon Strings on YYE.
Over the years I’ve seen people from other (used to be) popular forums saying they’ve washed strings to get them to feel fresh and new again. I personally haven’t done it before though.