How high is my risk of dead nipple (Necrosis?)

No one can give you a specific risk for loss of the areola. Breast measurements factor in as do other medical conditions such as diabetes. Patients who smoke Have greater risk. Previous pregnancies make a difference as well.

The nipple is carried on a pedicle of breast tissue following a reduction so by definition the blood supply to it is different following surgery.  In the hands of an experienced surgeon there should be minimal risk so I would suggest going on more then one consult to get other opinions if the first surgeon seemed concernedDr. Corbin

In a healthy young non-smoker, nipple necrosis should be very, very rare.  I think maybe a second opinion is in order.  Some of the newer techniques of breast reduction (vertical or central pedicle) are more friendly to the nipple in women with really saggy breasts and along distance from the nipple to the chest wall.  Your doctor may still be doing an inferior pedicle technique which cannot "carry" a nipple a very long distance.  These days, I am able to do reductions with maintaining nipple viability in ladies with really large and saggy breasts who years ago I would have done a free nipple graft on.  I have not done a free nipple graft surgery since I switched to the vertical reduction technique.  Breast reduction is a wonderful operation in young women burdened by very large breasts and usually the surgery goes very, very well. 

That should be extremely rare in a healthy young person unless you smoke. That would raise the risk considerably.