You never have to submit a WiFi rating. But when someone leaves you a rating, it's polite to return the favor. Basically, whenever I connect with someone here and it isn't just a drop in and leave to help them with their train station sort of visit, I leave them a WiFi rating just to show people they are a reliable person, since many of us will check those if we have to rely on them coming to our town and potentially mucking up a trade or sale or messing stuff up in our towns.
So, just whenever you feel like doing so, do so!
It's never necessary, it's just nice.