I didn't start with my "magnum opus" mostly because I don't think I am a strong enough writer to take it on yet. Heck my current main series was suppose to be something that is much more straight forward and I am using it to help practice constructing a narrative.
I think I would suggest to someone not to start with their magnum opus even if they are confident in their art. I think it is important to also practice constructing comics from narratives. I have seen comic with nice art but I have no idea what it going on because they haven't really nailed down comic presentation or dialogue writing. So starting with smaller stuff is helpful.