Even though food is more of a veterinary/medical issues, I often get asked about food and what to feed your dog so here is my opinion:

1) What do you eat? Are you a health nut? Do you buy organic, low fat, high fibre this that or the other thing? Then care about what your dog eats too.
2) Are you a couch potato that eats fried food, chips, and soft drinks as "part of a healthy breakfast"? Then don't worry about what your dog eats! You might want to worry about yourself first.

Basically I recommend using the same principles that help you decide what to feed yourself. Dogs and humans have evolved together for a very long time and dogs used to eat our garbage in a lot of cases. Generally they can eat what we eat. There are of course a few food exceptions that are not good for dogs but even these need to be consumed in fair quantities before they cause serious and immediate harm. For me and my dogs I generally like to know what I'm eating. If I can understand the majority of the ingredients list I'm pretty happy. Natural foods, minimal processing, minimal additives and refined products are generally what I look for, for both me and my pooches.

If you have hyper dogs, just like hyper kids, maybe lay off the high carbs such as corn. If you have an older dog that's less active, look for the lower calorie stuff. If your dog has specific issues however; allergies, skin conditions, bladder/kidney problems etc then you really should talk to your vet to get a proper diagnosis. Feel free to discuss with them what your food philosophy is so that they can prescribe what is good for the dog and good for your conscience.

As a trainer, I'm more concerned with "how" you feed your dog, not "what".