[Turtle91]

Adding all those tags will work but it is not considered "in good form"... and it is a whole heck of a lot of work...lol

Flightcheck is telling you it doesn't like <br /> all by itself...it needs to be wrapped in a different tag <p><br /></p> or <div><br /></div>...but there is a much better way to organize your paragraphs:

Let Sigil wrap each paragraph in a paragraph tag - that's what they are for. Don't use <br /> tags to give spaces between paragraphs.

Then you can style the paragraphs however you want using a CSS stylesheet. For example:

<p class="first">This is the first paragraph. It has a zero indent on the first line.</p>
<p>This is the second paragraph. It has a 1.2em indent by default.</p>
<p>This is the third paragraph. It has a 1.2em indent by default.</p>
<p>This is the fourth paragraph. It has a 1.2em indent by default.</p>


CSS:
p       {text-indent:1.2em}
p.first {text-indent:0}

You can further style your paragraphs with extra space and no indent (eg a scene break in a chapter) by giving it it's own class:

<p class="SecBrk">This is the first paragraph after a scene break. It has a zero indent on the first line and extra space before the paragraph.</p>
<p>This is the second paragraph. It has a 1.2em indent by default.</p>

CSS:
p.SecBrk {text-indent:0; margin-top:2em}

The final product would look something like this:

<p class="first">This is the first paragraph. It has a zero indent on the first line.</p>
<p>This is the second paragraph. It has a 1.2em indent by default.</p>
<p>This is the third paragraph. It has a 1.2em indent by default.</p>
<p>This is the fourth paragraph. It has a 1.2em indent by default.</p>
<p class="SecBrk">This is the first paragraph after a scene break. It has a zero indent on the first line and extra space before the paragraph.</p>
<p>This is the second paragraph. It has a 1.2em indent by default.</p>
<p>This is the third paragraph. It has a 1.2em indent by default.</p>
<p>This is the fourth paragraph. It has a 1.2em indent by default.</p>


CSS:
p        {text-indent:1.2em}
p.first  {text-indent:0}
p.SecBrk {text-indent:0; margin-top:2em}

In my opinion you should define the default paragraph the way you want the vast majority of your paragraphs to look, and save the specific classes for specific paragraphs that look different. That means a lot less work for you when you are creating the epub, and a lot less work for the reader/app when it is trying to display your book (thus it is a smoother/faster experience for the end user.)
Cheers!