The way to do it is to have the cleanest CSS. You don't want it too complicated or with extra bits that you don't need.
I've seen lots of CSS from commercial eBooks that has lots of extra rubbish in the classes that are being used. If I bother to strip out the rubbish, I end up with pretty good CSS. The class names can still be rubbish. The class names from In Design are not nice and sometimes there are multiple classes used that could be changed to be just one class. But I'm too lazy to do that.
I don't do the P + P as I do like to see <p class="noindent"> so I know what's what by looking as the HTML code. It's amazing how easy it can be to take an ePub that's not nice and make it nice. Now if only I could get paid for it. I'd be happy to take the finished ePub and make it even better. I could make it smaller, better code so less change of causing a glitch. Sometimes these overly bulky CSS files can cause some programs to not work well.