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.

The answer to your question really requires knowing exactly where and how much cartilage your surgeon is considering removing (or shaving). As long as enough cartilage support is maintained and the area to be shaved is fairly tiny this shouldn't be a major concern, but you may want to get a couple more opinions from surgeons experienced in revision rhinoplasty first. As is often the case in rhinoplasty, there may be more than one way to safely achieve the look you desire.All the best,--DCPPearson Facial Plastic Surgery®

Without pre-op and post-op photos- all views, reading the op-report, and of course the benefit of in person exam- it's difficult to give the best answer. Revision Rhinoplasty is very challenging and specific type of procedure requiring clear vision of the result and ability to achieve it.If you are seeking a second opinion - I suggest that you see an experienced Board Certified PS with extensive Rhinoplasty experience for evaluation .

Without a full set of profile, full frontal photos, it is hard to determine what your best treatment options are and if there is any cause for concern.  I recommend getting a second and maybe third opinion from a few facial plastic surgeons who specializes in revision procedures. Look through before and after photos from the surgeon and make sure you like the results he/she has achieved.    Request  computer imaging so that you can get a good idea of how your nose will appear after surgery.  This ensures you and your surgeon are united in terms of the end result.See the link below for more on Nasal Bossae and possible treatment options.  Keep in mind each case is unique so treatment options are not applicable to all Nasal Bossae issues.Best,Dr. S.