I think you need to examine your EPUB goal
If you are making a book for (retail) distribution, I believe the CSS route is best as it allows quick, one place, style edits.
Then there is the choice of CSS methodology:
Simple: Style by tag alone. (IMHO never leave a style 'default' or you are at the mercy of the DEVICE. Not all devices give control to the user. I have 6 text sizes. That's it)
h3 {font-size: 18pt } << the down side is ALL H3's are exactly the same
p {margin: 0; text-indent: 1.2em}
or
Full control: using selectors
<p class='nonindent'>
.nonindent{text-indent: 0}
Reverse engineer a few books you like the looks of