It is not the crown itself that can cause the bad breath. If the crown shape traps food you may develop an odor around the crown if your home care isn't sufficient to keep it clean. A waterpik works great for cleaning around crown and bridge work. If you are having odor and you are able to keep it clean, call your dentist and have him check it. If there is leaking, you could be developing decay around the crown.
Having crowns should not cause bad breath as long as they are well maintained and kept clean; unless your crowns have leakage, poor fit or cavities around them. Please ask your dentist or hygienist, they can show your proper way to keep them clean. Good Luck!