Virtually every plastic surgeon will tell you that the maximum height of dissection in a tummy tuck is to the xiphoid (very lowest tip of the breastbone), and even lower to the sides (usually well below the ribcage). This is to allow maximum loose skin removal and abdominal skin tightening while still preserving adequate blood supply for healing. (This is also why it is critical for tummy tuck patients to avoid all tobacco or nicotine products, as they are potent vasoconstrictors that can lead to dead skin or belly button.)
Thus, there is a "zone of adherence" in the upper abdomen that prevents any stretch or distortion of the breasts from the tummy tuck, no matter how tight the repair is! There can occasionally be some initial tightness in the skin just below the breasts, but this settles and softens as the tummy tuck heals over several months.
So, whatever you breasts look like prior to tummy tuck, unless you have breast surgery at the same time, your tummy tuck will not affect the appearance of your breasts.