Originally Posted by Man Eating Duck
Headings really should be h-tags, as they have semantic meaning in addition to formatting. We create academic books which usually have three or more heading levels , so this might be more important to us than for a straight novel.
I agree! To be clear, CS5 and up allows one to define the HTML tags to be used for a given style upon export to epub. CS4 does not allow this; the styles get exported as <p class="whatever> blocks. But it's easy to change them to <h tags in Sigil. That's what I meant above.