I almost always post anonymously. This is mainly because I just want to share my opinion and leave it at that. I don't want pm's, I don't want people to be biased towards an opinion I share if they didn't agree to a past opinion I shared, and I don't want to feel obligated to reply to people (mainly because I don't want future people to recognize my name and be like "wow that's the person who didn't respond, I'm going to dislike their opinion"). Also when I post a comment on a sex related topic I don't want a bunch of creepers with "similar interests" messaging me.
I never have and never will post anonymously. Stand by your opinions, if they have value, and don't be a coward. The only reason why it would be a reasonable option would be if you were divulging private information about yourself which I rarely, if ever, do on this website.