how soon for 1st echo & tamoxifen ?

Hello everybody! I have had 8 weekly herceptin treatments. I continually worry about how and if it will affect my heart. So, my question is how soon did most of you get your first echo? Would getting one after just 8 treatments be too soon? My next question concerns tamoxifen-I'm 36 and borderline(11%) estrogen + and progesterone -. I wondering if tamoxifen is a good choice. I'm confused if whether or not it works well with herceptin and especially in my case when I'm only borderline positive. I appreciate any thoughts, advice or opinions. Thanks so much!