i have the h&r cup kit, and have 18x7.5 wheels with a 40mm offset (wrapped in 215/35). never had the problem with the same kit/wheels on my zx3, but i guess the added hundred pounds or so in the sedan is JUST enough to give it some problems. it's only over big dips, or under hard cornering. it's still annoying as hell. ha ha
if you are careful, you can avoid cracking by applying heat to the area....just don't go crazy...a heat gun on low should help you out. If you don't wanna use a bat, you can always buy the rollers that are meant for the job.