Creating a topic seems a little hard because it needs to prepare a lot to have an original one, not inspired by existing threads. So participating in topics differs from create new ones.
It's not a must that you are supposed to be involved in any discussion that is going on in your community. In a situation whereby it's something that you are not comfortable with, you can simply stay out of it and nobody can force you to contribute in that kind of topic.
It varies. When I was younger and first starting out, I likely should have bowed out of conversations. I don't know if I have necessarily gotten wiser with age or more apathetic. There are certain things I will always tackle head-on no matter whether it is the smart thing to do (I will always fight against prejudices and / or someone using a belief system to justify said prejudices), but, if it is something small that means nothing to me and a lot to somebody else (and it is harmless), I will let it go.
We have a team of 13 mods/admins on our popular forum so it's not necessary to participate in every discussion. I can choose to participate in the ones that interest me. There is no way I could ever keep up with all the posts by myself. I usually read the Starting Post in a Topic to make sure it's been posted in the right forum, but other than that, I read and post on what I want to.
I rarely bow out of topics, especially if I think the topic is heading in the wrong direction. I prefer to try and steer things back towards the middle ground.
I try my best to not ignore threads, especially in a mod/admin role. Normally in the earlier stages of discussion, I'll try and hop in to steer things back in the right direction or promote debate in a healthier manner. If it reaches a stage where I feel the thread is 'unreplyable' due to being so off course, that's normally a good indication to take moderation action; The last thing you want is numerous threads turned toxic, it's very off-putting for newer members