Trust levels essentially act as a safeguard for both your new users and your community at large. A good analogy to understand them is imagining they are a stranger in your home. When they first visit you don’t know them well enough to trust them so you wouldn’t let them walk around unsupervised. And from their perspective, they haven’t been to your home before so they don’t know their way around.
Trust levels are a way that you can safely allow someone into the community in a sand-boxed way so that they don’t have access to functionality which could be problematic. They can’t PM strangers, they can’t drop spam links etc. As they spend time reading and engaging, they prove they are there for the right reasons so they get more functionality. They are now familiar enough with the forum to understand how to use it.
So leave those in place with the default settings, but remember that you can tweak those later if necessary. Then build your custom groups on top, which will dictate who can access what content on an holistic level.