RATIONALE: Drugs used in chemotherapy work in different ways to stop the growth of tumor cells, either by killing the cells or by stopping them from dividing. Giving more than one drug (combination chemotherapy) may kill more tumor cells. Estrogen can cause the growth of breast cancer cells. Hormone therapy using tamoxifen may fight breast cancer by blocking the use of estrogen by the tumor cells. Letrozole may fight breast cancer by lowering the amount of estrogen the body makes. Giving chemotherapy and hormone therapy after surgery may kill any tumor cells that remain after surgery. It is not yet known whether genetic testing is more effective than clinical assessment in determining the need for chemotherapy in treating breast cancer.