When you're cooking conventionally, it's important to remember that white meat cooks a bit faster than dark meat. When you're using a slow cooker, the difference between the parts is so negligible that they are not differentiated.
One thing to keep in mind is that bones make a difference, and boneless pieces, be they breasts or thighs, will cook faster than those with bones.
You might wonder why you're instructed to use chicken pieces in the recipes rather than plunking the whole bird into the slow cooker. The reason is because food in the slow cooker cooks from all sides, and the smaller the pieces, the faster they will cook through. A whole bird should only be cooked on High because its sheer mass keeps it in the bacterial danger zone too long. That's also the reason frozen chicken pieces should never be placed into the slow cooker until they are thawed.
Merrily We Roll Along
One way to serve chicken that is always visually appealing is to pound the breasts thin between two sheets of plastic wrap, layer them with a filling, and roll them tightly. You can keep them closed with toothpicks or kitchen twine. When the chicken is cooked, these rolls get sliced into medallions. Anything from a few leaves of fresh spinach to slices of ham will work well as filling, and the rolls cook in the same time as chicken pieces with bones.