My thoughts are that any supplements are only beneficial if you are deficient in those vitamins or minerals. You can have your levels checked to give you an idea. I have been on Letrozole for just over 3 years. I had my Vit D level checked a few times and it has been fine.
As for calcium, the new recommendations are that it is far better to get your calcium through a balanced diet (because of the issue with supplements and heart issues). I see a dietician as part of my monitoring of bone density. She has helped to guide me to ensure my calcium intake through food is enough for me. As I understand it there is more to bone health then just Vit D and calcium. Magnesium and Vit K are also important for example. Weight bearing and impact exercise is also very important.
I am more comfortable with the idea of eating well to ensure adequate levels and getting a little sunshine each day for my Vit D. But some people need supplements if they have issues such as dairy intolerance etc or their lifestyle limits their ability to get enough Vit D.
I hope this helps you work out what might be the right thing for you. I hope that the Letrozole goes ok for you. Take care. xxx