When mixing different foods you run the risk of your dog getting either not enough or too much of certain nutrients.
This misconception seems to originate from dog owners feeling guilty about the type of food they feed or those who follow their own ideas for "improving" their dog's diet without doing any research. Every brand of dog food is following a specific formulation and philosophy developed by the manufacturer. They are all formulated to supply a balanced amount of nutrients in a ration of a certain size, based on the body weight of the dog. Last but not least, if digestive upset occurs, it's going to take so much longer to figure out what exactly caused it compared to just eliminating either the commercial food or whatever extras were fed recently.
If you want to offer more variety, stick to one line of food of the same brand at a time and rotate between brands every few months.
**Supplementing the dry food with fresh, "live" foods like vegetables, fruit, yogurt, meat or a bit of canned food is also safe and healthy.