Are tissue expanders always required?

The short answer is no, expanders are not always required.
However, there are factors that might lead your surgeon to favor an expander and then an implant such as the quality of your tissue (remember that a lot of the support structure of the breast is removed in a mastectomy), and the position of your current breast mound in relation to your chest.
Often, if all factors are favorable, excellent results can be obtained by placing an implant immediately. But, not always. I would recommend you defer to your surgeon as to their level of comfort with going directly to implants.

This is my favorite question, because many doctors don't do lots of direct to implant breast reconstruction, especially not for D cups. As the implant is bigger it can get technically more difficult. Then they tell the patient that they are "too big" to go to implant, and require a tissue expander, which is ridiculous. Your answer is that you have to look around for a doctor who DOES do direct to implant in one stage routinely. I participate in FORCE, which advocates for woman with genetically high risk for breast cancer, who can help find you docs in different area of the country.

I'm sorry to hear you are facing this decision. The answer is, no, you do not always require tissue expanders prior to true implant placement. Some situations allow for the implants to be placed at the time of mastectomy, avoiding the expanders. However, you have to be just the right candidate for it, and it still carries a reasonably high rate of requiring revisionary surgery at a later date if you are not happy with the shape/size. Best wishes.

Dear Mack:
Not all patients that undergo mastectomy require tissue expansion. Some patients are candidates for direct to implant reconstruction, but usually these are patients with A/B or small C cup size breasts.
With a full D size, I am skeptical you are a candidate for either nipple sparing mastectomy let alone direct to implant reconstruction. 
Although it is difficult to say without photos.

Best of Luck

Dr. K