arfellian, I'm probably misinterpreting what theducks is saying, but at the risk of doing so, I think it's important to clarify something.
First, the problem for me was caused by failure to include an extra paragraph return in the original .doc file between the main body text and the start of a centered warning notice. I'd used one at the end of the warning notice to separate it from the following main body text, so it appeared as I expected it to.
The point is that if I'd done it correctly in Word, it would have appeared correctly in the epub created with Sigil. Perkin helped me understand that an extra paragraph return added in Sigil book view wasn't fixing the problem and why. As DaleDe confirmed, using the correct code made sure that the rendering engine interpreted the blank line the way I wanted it to. In other words, "leave it alone, it's intentional."
In my rather limited experience, single paragraph returns added to the Word document before conversion have always appeared correctly in the .epub. But if I try to use more than one return for additional space, things get ugly in a hurry.
theducks will correct me if I'm wrong, but I'm under the impression that it's impossible to eliminate orphans in an ebook, if by orphans we mean single lines of text sitting at the top of an otherwise blank page. You can go to the trouble to do that throughout the book so they don't appear in ADE, and that .epub viewed on an eReader, especially with the reader's ability to adjust font size, will appear very much differently.
But regardless of the font size used by the reader, you can avoid the problem of having a chapter heading, for example, sitting at the bottom of a page and the text that follows beginning at the top of the next.
I personally like the look of page breaks at the start of each chapter rather than having the entire book appear with continuously flowing text. I build my .doc file with the chapter headings in the Heading 1 style and use something like 48 points before and 24 after in the paragraph spacing portion of the style definition. This sets the chapter heading apart so it clearly telegraphs its location.
I used to manually insert page breaks in Word to begin the each chapter on a new page, but the better way is to build the .doc with no page breaks between chapters and add them in Sigil with the chapter break command. This has worked flawlessly for me, and I know that the chapter heading will always appear at the top of a new page with the accompanying text below it.
I know very little about code, so my solution is to carefully edit the .doc file to avoid the trouble makers like multiple extra returns, tabs, extra spaces between the end of sentences and the paragraph returns, etc. If I do that, the Sigil-to-epub conversions usually appear as I expect. If I need more spacing than provided by single returns, I create a style and use the before and after spacing values to provide it.
I apologize if I've jumped in here with a comment that misses the point, and I don't want to appear as if I'm contradicting the experts. That is most certainly not my intent.