Whatever your definition of tithe, I think it implies more than simply appealing for donations with no sanction.
In some European countries churches have the legal right to tax their members, which is ollected along with general taxation. So far (as far as I know) JWs have not joined such programs, I don't know if that was on principle or because they would be rejected. Maybe they will try to be included in the scheme at some point.
In any case JWs are very low contributors from what I can tell. I am looking at local churches that generally collect £30, 000 to £200,000 a year. The local congregation of JWs less than £9000.