I would ask the surgeon performing the sinus surgery. If they wanted you to hold off, or stop it would likely be due to bleeding risks. At least that was what my top surgeon requested and why. It seems to be hit or miss though. More and more surgeons aren't required guys to go off T for top surgery. My surgeon for my hysterectomy told me she wanted me to keep taking it to regulate my hormones post-op.
T doesn't change your bone structure, but your fat placement in the face. So I wouldn't imagine anything would be affected by surgery. If you've already started making progress on starting T, you could also ask the doctor who would be prescribing.