Is there a rule that says the first paragraph of a new chapter should NOT be indented? Or is the decision to indent or not indent the first paragraph solely a case-by-case design decision?
You get to do what you want with that indent. Setting the first line flush left for a chapter-opening paragraph (and after space breaks within the text as well) is the general “norm” when it comes to interior text design, but there’s no absolute hard-and-fast rule. My designer colleagues tell me it’s nice to have the freedom to play with this element—they’d put a big ol’ indent if that worked for a project’s design style—but you’ll almost never see examples of that because breaking from this norm can strike readers as awkward. Keep that in mind if you’re designing the interior for your own self-published project. If you’re just wondering how to format your manuscript for submission, go with the tried-and-true flush left treatment for those chapter and break openers. Definitely don’t submit manuscripts set in that blog-like style that has extra line spacing between paragraphs and all first lines flush left.