Short article from The Washington Post on pet food myths outlines several points, including that "grain free" is a marketing gimmick and that "raw" diets pose additional contamination dangers (never mind that your domesticated dog/cat isn't "wild" and likely has different nutritional needs).
I would add two general rules of thumb: 1. if a food has "wild" in its name, you're likely overpaying for something you don't need, and 2. almost all ingredients below the 6th ingredient in an over the counter pet food are in usually insignificant amounts; included only to make the pet owner feel better about what they're feeding their companions.
Washington Post article
