I will be getting Tamoxifen after all the chemo is done. I think you should take tamoxifen because it's proven to work in preventing cancer.
Tamoxifen works on women whose tumors had been estrogen and progesterone positive.
That means that the tumor was based on excess estrogen. HER2 is over expressed monoclonal antibody which actually makes tumors grow faster. But excess estrogen actually makes the tumor. So, the way I see it, you can't afford not to take Tamoxifen. The only drug designed to fight specificly HER2 is Herceptin currently available in clinical trials for prevention of recurrence.