You should always direct people to a landing page where they can choose from or be directed to multiple different instances.
https://kbin.pub/en or https://kbin.fediverse.observer/list
https://join-lemmy.org/ or https://lemmy.fediverse.observer/list
You can direct people away from specific instances if, for example, they have been de-federated.
Absolutely agree. I totally get the idea of federated content and letting people make their own decisions, but the higher the barrier of entry, the less mainstream the fediverse will be. Especially right now, people are going to want an easy replacement for reddit, not learn a fundamentally new system that is still in the early adoption phase by tech people.