The above is a good and safe default, so I repeat, don't do it! At least, unless you have a good reason and know what you're doing. I'm serious, and in most cases you really don't know what you're doing.
I'm currently rescuing an instance that is a mess, because I couldn't say no. And I'm sure there a many more instances like this out there. Instances that have grown too fast and are maintained by a single person. THIS could break the fediverse.
So, here are my tips:
+ Don't run an instance for people or a community you don't know.
+ Don't run an instance if you don't have admin experience yourself and don't have a motivated and experienced friend who wants to run the instance together with you.
+ Don't run multiple instances.
+ Don't run an instance for more than 100 people if you are a single admin without admin experience. (If you still think it is a good idea. And a single user instance can be a good idea, for example.)