The humour thing is definitely not mandatory, I sometimes do it when I feel like it (or when doing stand-up comedy) but not all the time. Regarding mentioning it though -here is my own experience: I would hold back from speaking because I was afraid of stuttering. I have found that by bringing it up, I allowed myself to relax, and to feel significantly more willing to talk (as opposed to worrying about getting stuck and receiving odd stares). I mention it for myself, not for others. Personally, it's beneficial. We're all different though!