Just want to add that there are plenty of studies that show that diet does make a difference in the amount of tartar on teeth. Canned foods are considered the worst because they are often sticky. Here's one study:
Periodontal Disease and Diet in Domestic Pets Toothbrushing is the best choice, but the study found that "the best way to maintain healthy periodontal tissues in our pets is frequent toothbrushing. The use of a dental hygiene chew and/or a diet designed to reduce dental deposits can be useful adjunctive measures and should be recommended."