We feed pre-made raw and we mix brands and proteins. They get one brand at breakfast and lunch and another brand at dinner. This is because they seem to loose weight on the one brand and gain on the other. So the combination helps them maintain a consistent weight. We switch proteins at each bag (about every 1 1/2 - 2 weeks) to give their bodies variety in which protein they have to process. I'm sure it's nice variety for their taste buds too, but that's not the main reason we do it. I'm not sure they take long enough to eat to actually taste anything.
From what I understand, most do not recommend changing food frequently as Yorkies tend to have very sensitive tummies. My understanding is that dogs on a raw diet can tolerate variety in raw proteins better than with other types of diets.