Have you seen the Diet forum,
Yorkie Health & Diet - YorkieTalk.com Forums - Yorkshire Terrier Community. I agree with your vet, and as long as you are not buying the very cheapest of foods, you'll probably be all right, but there are many different opinions on what is the "best" food. The information can be confusing, diet is usually more important if you dog has allergies, or a problem digesting certain nutrients, and then it can be much more difficult to select the right diet. Petsmart does offer a refund on any food you buy, if your dog has difficulty with it, or just won't eat it; so keep those receipts. Joey wouldn't eat most of the higher rated foods, and so we decided on a medium rated food. He seems to be dong well. You can't put too much confidence in the ratings, there are only reviewed by one person, and they don't say their credentials, so sometimes I don't feel like I'm being giving the full picture of the food.
Some of the things I considered when choosing a brand was convenience to purchase, did it have to be ordered or could I buy it locally? Nutrition, of course is important, and I have a little background in nutrition so I believe I'm able to understand some of the facts, and choose a food with ingredients that I believe to be healthy. The manufacture and their reputation, is important - how are they on recalls, how long has the company been around, and how big of company? I like a company that's large and has been around for a while, because if something does go wrong with their product and they have to recall it, you are more likely to hear about it fast. Another important consideration was did Joey like it, would he eat it? So many of the brands he just would not eat. These are some of the things you might wish to consider when buying a food. Remember when offering a new food go slowly, just give one part of the new food mixed with three parts of the old food, and the next day give half and half, and if the dog is experiencing no tummy upsets, you can give them 3 parts of new food to 1 part of old on third day. On the 4th day, it would be all new food. Cut back and go slower, if there are any problems. If they pick out only their old food and leave the new, you might have to find a different brand. Best of luck, and try not to go nuts!
