[…] treat each Lemmy community as a community, not an audience.

I think it depends on the community in question, and the nature of the post. If, for example, one is looking for an answer to a question, or help with something, I would argue that one would, generally, want to target the largest relevant audience to maximize the surface area of potential people who can help. At any rate, more specifically, I don't think it's one or the other, but rather both — one would want to find the largest and the most relevant community. By my experience, another common behavior is to cross-post to multiple communities. This seems to be especially more common in a federated forum like Lemmy where there could be any number of duplicate communities.