Hi William and welcome to our community! 
This is a great question! The answer depends on your organization size, how much you trust your colleagues, and what you are using your discourse site for. @Heliosurge is right that groups and secure categories are an important part of it.
For an internal company site, I’d recommend not tinkering with the default trust level settings at first. It can become overwhelming because there are many of them, and they are designed to be safe and work well for both public and private communities of all sizes (see Trust Level Permissions Reference to see what they each can do).
I’d also give everyone TL4 which lets them edit posts of other members and do alot of moderator actions. If you’re not sure, you can start out with TL3. As you move along and people bump up against limits that hamper their work and ability to collaborate, you can then give TL3 or TL4 more privileges. Use groups to limit access to some categories as needed. Keep the site feedback category so there is an outlet for giving feedback and asking for changes.
We use Discourse internally too (of course!) and have set all users at TL4, and gave TL4 access to additional moderator privs like deleting and merging topics. Only certain high level managers and our IT department have admin or moderator privileges. We’re now 100+ employees in the company.. when we were smaller, everyone was admin. Needless to say, there is a high level of trust on our team! We use it as our workplace, doing everything e.g. talking strategy, collaborating on projects, weekly updates, sharing jokes in icebreaker topics, and so much more. Everyone helps garden discussions, deletes unwanted topics and posts, improves topic titles, moves topics to better categories, closes topics when they are complete, etc etc.
But at the same time we are able to limit access thanks to groups. Nobody needs to be able to see the personal messages of other colleagues they are not involved in, and we can have secure categories for e.g. management discussions.
We have an onboarding procedure that we have every new employee complete which introduces them to all of this. They get TL4 access on day one and it works great! The risk is low because it’s easy to see changes that have been made, actions are logged, and experienced colleagues give tips and guidance to new folks. Everyone quickly becomes an expert at Discourse!
To take another example, I once worked for an organization that had a community with mostly public categories. Anyone could join and move up the trust level ladder as usual, without any change from the out of the box settings. If they were invited they’d enter as trust level 1 as usual too. We opted to give all employees trust level 2 as they joined, and let them work towards trust level 3 as any member could. We had a small moderator team that were given moderator privs, and one admin (me).
We had two main secure categories, one for employees and one for management. We also had working groups that also had their own secure categories. We’d give access to those categories as appropriate using discourse groups. As admin, I muted the management category.
I’d say this setup worked reasonably well for the organization where there was no expectation that employees become Discourse experts. They could just participate in discussions like any member, and use the flagging system to ask moderators to do any gardening/moderation tasks. On the other hand, there was some tension among those employees who wanted to be able to do more and were not allowed to, and created extra work for the moderator team. The discourse site was also not the only workplace tool for the organization, so the internal categories never got very active.