Revision rhinoplasty to correct bossa of tip, surgeon wants to shave down the cartilage. Should I be concerned?

You are correct in that often the preferred way to correct tip boss is to bring the two intermediate crura comprising the dome back together or using a graft to camouflage the bossae. That being said there are other ways to correct it such as how your surgeon suggested as other situations can cause the tips to be asymmetrically narrowed so I would ask your surgeon to explain it to you better perhaps with a picture to demonstrate what he/she wants to do. Good luck with your surgery.

Without pictures it is difficult to advise and actually even an examination may not give the full answer.  You are correct that the tip cartilages have to be of adequate thickness and strength to support the nose, but many times selective shaving/and or scoring or suturing can correct boss’s.  Sometimes bringing the cartilages together is the right thing to do and sometimes combining scoring or weakening with an underlay cartilage graft is the best approach.  Reopening the nose and evaluating the tip cartilages at the time of surgery will determine the best approach.

But you didn't post any photos, and that makes it hard to give good advice.
But yes, bossae usually result from too-tightly-curved tip prominences that have migrated away from each other. Often, there are associated problems, like a nose that is still too projecting, or too long. 
Revision rhinoplasty is an exquisitely difficult operation to get right, and you should only have surgery if you are able to make yourself very confident in your surgeon's skills. For most noses, it's much better to not have surgery than to have inexpertly-performed surgery. Read my essay on how to stay out of trouble while selecting a rhinoplasty surgeon, and how to tell whether your first surgeon should be performing your revision.