Agree rendering is perhaps the most important feature and many many apps, usually those that provide options to mess with the CSS, mess it up in someway.
Did a quick test of one ePub in MapleRead iPhone. Sadly it didn't do well. From what i remember of the CSS, it was a "p + h3" margin-top. But that looks to be the only issue with that one ePub so not too bad and better than most. Options to adjust paragraph indents and spaces between paragraphs maybe should just not exist since so far, I don't think I've seen one app with those options without problems.
I did find one other issue. For chapters that span multiple files, it's perhaps best for the scroller to show a heading for next files until it finds a new heading. That might not be best for front matter that might not always have a TOC entry but i believe is safest overall.