Every ePub display engine has its own quirks. If you want to see how your books will look on an iPhone, the only way to be sure is to look at them on an iPhone. And even then the results will be app-specific. There are at least half a dozen different apps that the user could be using to display books on their iPhone, and the book will look different in each one.
By far the most commonly used ePub display engine used in bookreaders is Adobe Digital Editions (ADE). I'd recommend that you use that as your main test environment.