This is a layout issue.
What the agent or editor wants might not be the same convention that the layout ultimately uses. Looking in published books might not give you what they want. And different folks might have different preferences. While cruising agent and publisher submission guidelines last year, I remember very few that specified a convention.
In the olden days of the typewriter, we used only an extra blank line to separate scenes. The lineotype operator kept it that way until a scene break and page break coincided, something we couldn't predict by looking at galleys. When that happened, the operator inserted the publisher's convention at the bottom of the page, often * * *.
I do not miss the olden days of the typewriter.
Another convention is to indent the first lines of all paragraphs except the ones that open chapters and scenes. Those paragraphs have the first line flush left along with the rest of the paragraph.
In Word I use a normal paragraph style for regular paragraphs, a firstparagraph style for scene and chapter openings, and a scenebreaker style for whatever separators I need to use. That makes it easier to reformat a manuscript according to the specifications of the recipient.
Today, with e-books having different page breaks depending on the reader's preferences, you need some kind of separator. I found a nice ornamental symbol for that, which looks nice in a print edition too.
For e-books, don't use extra blank lines for anything. They get eaten in the conversion to mobi and epub. As do double-spaced formats. Use your word processor's paragraph spacing settings in the style to make extra space between paragraphs.