If your breathing is normal then no surgery on the septum is indicated for breathing problems. The septum is the best place to harvest cartilage grafts for columellar struts and various tip grafts when needed. A small portion of cartilaginous septum is harvested, leaving the lining intact, the grafts are cut out of a pattern on the piece or removed septal cartilage, and any excess is put back. This should not interfere with breathing. The risks of taking septal cartilage include septal perforation, which means the site of harvest of the cartilage becomes a through and through opening in the septum, without intervening cartilage. This hole can be corrected if it is symptomatic, e.g., causing crusting or a whistling sound. Another risk is removing excess septal cartilage so that the dorsum of the nose loses support. Both of these complications are uncommon.
Granted that you have a skilled and experienced plastic surgeon operating on you, I would not be overly concerned about getting a complication from harvesting septal cartilage.
Other potential causes of breathing difficulties after cosmetic
rhinoplasty are collapse of the internal nasal valve which can occur when the dorsum is reduced and the upper lateral cartilages fall in. This can be prevented with septal cartilage spreader grafts, turnover upper lateral grafts and/or by suturing the upper lateral cartilage back to the septum. External nasal valve collapse (the nostrils) can be caused by excessive reduction of the alar (nostril) cartilages during tip shaping, and failure to compensate for loss of tip support or alar rim support with either suture support or graft support to the alar cartilages or columella.
This technical answer is presented to demonstrate that rhinoplasty/septoplasty is a technically intricate operation, but that the problems than can occur are known and preventable, and that preventing and treating them are a part of the repertoire of
rhinoplasty surgeons doing these operations today.