Long story short, you should use TOR to maintain anonymity online.
The challenge that this faces is the fact that many sites will restrict/block access from TOR exit nodes. These forums will even make you pay a (very) small amount of BTC in order to access your account if you try to access via TOR (AFAIK this is only a one time payment).
I have seen sites that do not even allow users to register or visit using TOR. But these are not .onion sites but the regular web. I think the downside of TOR is that the subset which uses it is much smaller than the total set, thus making it easier to find you for an ISP, correct me if I'm wrong.