A third party iPhone charger, approved by Apple will be just as safe to use on your iPhone as the charger that came with the phone. There are, however, unlicenced chargers that can harm your phone and be dangerous to the user.
The Chargers and cords you get from your cell phone carrier are more than likely to be licensed and certified by Apple. It's the ones you pick up from eBay or your local 7-11 are the ones I'd be wary of...