I don't wifi with very many people and even fewer have left me wifi ratings. Some people utilize the system while others ignore it. Although it is polite to leave a wifi rating (whether good or bad) to let others know how a person acts via wifi, it is not an obligation.
Before you wifi with someone, be sure to let them know that you will be submitting a wifi rating for them and would like them to extend the same courtesy.
Ever since it was put into place, I always rate everybody now. Unlike eBay's feedback, TBT's feedback thing only takes like 10 seconds. Seriously. There's no reason NOT to do it.
Even though I now rate everyone, I have had people not rate me. To be honest, it doesn't really bother me if they don't. I don't think the WiFi rating system is as useful as it could be. There's only like 80 characters that's visible to the general user populace so no room for details. There's also no way to mark a person as someone you consistently go online with and *in your opinion* is trustworthy.
