Continuing from your questions on the Sigil forum, a bit more appropriate here. I use something along these lines in extra css:
Code:
p {margin-top:0.3em; margin-bottom:0.3em; text-indent:3%; text-align:justify}
h1+p, h2+p, h3+p, p.softbreak+p {margin-top:0.1em; margin-bottom:0.3em; text-indent:0%; text-align:justify}
That sticks indents everywhere except for the places you'd not want them for commonly formatted books.
I never noticed a problem with these not working, but perhaps I haven't tried with ebooks where it needed to be over-ridden for some paragraphs, not sure.