the sides the lowest on the whole kit. and i don't think just the rear bumper would flow correctly with the rest of the car stock.
mine cracks almost all the time and my front bumper, then again my car used to be slammed. but at stock height, honestly you shouldn't have a problem or worry about anything. just drive safely over bumps other than that it doesn't hit or scrape at all. as of now I haven't had any problems of scraping or hitting my sides after i raised up my car. hope that helps.