Not absolutely necessary but rather a safety net. If the fuel/timing isn't perfect, then you run the risk of detonation which can bust the stock cast pistons pretty easy. Forged pistons can take a little more abuse, but you still have to be quick to back off the throttle if you hear pinging. The more boost you run, the more the risk so forged pistons are not a bad idea. I think RabidChimp was offering some forged pistons. Can also do a web search for J.E., Ross, Wiseco and possibly a few other brands.