I use to always mix different brands together and thought it was healthy and the best thing to do until recently.  I read an article from the Dog Journal and it's lists all the top dog foods for overall health and wellness.  In addition, it lists the food we should definitely avoid.  But what I found really interesting was that we should "Switch, But Don't Mix" food.  I will type out just word for word a small part of what I read:
 
Article Title:  Switch, But Don't Mix - Why You Shouldn't Seek A Sole, "Perfect" Food For Your Dog  
"I suggest that they give their dogs one food for 2-4 months, and then switch to another food, and then another.  Ideally, the foods are made by a few different manufacturers and contain completely different protein sources, too."
 
"To prevent nutrient toxicity, deficiency, or imbalance, the simple solution is to feed a variety of high-quality foods - perhaps three or four foods over the course of a year."
 
"There are other reasons to change a dog's food every few months or so.  Switching from one source of protein to another occassionally can help prevent the development of food allergies or intollerances.  It can also prevent a dog from developing a stubborn preference for just one kind of food, which can be highly inconvenient."
 
"Other than when you are switcing from one food to another, it's not a good idea to feed different foods at the same time.  Your dog might enjoy a mix of half this and half that, but if he suddenly exhibits a digestive problem, it will be harder to track down the offending ingredient." 
I wish I could copy and paste the article for everyone to read -- it's an AMAZING article and so informative -- but it's scanned on my computer as a PDF and I don't know how to convert is to text so that I can copy and paste it here.  But if anyone would like me to forward them a copy of the article, just PM me your e-mail address and I will be glad to!  I learned so much from it and I'd be happy to share it!! 
PS - It also gives a detailed description of dog food ingredients, who approves the food and who is reputable and not reputable, nutritional profiles, feeding trials, canned foods NOT recommended, approved healthy canned foods, commonly used canned food additives (a real eye opener), how to calculate "dry matter" or actual nutrient levels, not recommend hard food, how to pick the right food for YOUR dog, a list of approved healthy hard food, recognizing good and bad ingredients, organic products, etc. and much more!! 
So if you want me to e-mail you this article, just PM me!