Dry food and tartar control treats do not clean the teeth. Period. The dry food myth is perpetuated by pet food manufacturers, IMHO because they make a much higher profit margin on dry than canned. Many studies have disproven it. The FDA considers tartar control treats "misbranded," that is, falsely labeled, but it's a "low regulatory priority" because they have better things to do than enforce it.
If your cat has stinky breath, it may be time for the vet to have a look. Most cats need a professional cleaning every year or two, if you're not brushing at least 3-4 times a week. Tartar build-up and decay can occur quickly and cause considerable problems if left untreated.