Markstudy - I had a very similar dealership experience.
I had the TSB performed on my Si this summer as well. The 3rd gear "grind" was hardly noticable in my car, but the service advisor agreed to have the TSB performed anyways.
My car had never popped out of 3rd gear, nor did it severely "crunch" or grind going into 3rd gear. The transmission didn't display any of the "major" symptoms of being in need of the TSB. The service advisor mentioned that cars that experienced those types of aforementioned symptoms were the ones that could *really* benefit from having the TSB performed.
FWIW, after the TSB was performed, I noticed slightly smoother shifting going into 3rd. Was it necessary? Probably not, but at least it's done with. Plus, if I ever decide to sell the car down the road, any prospective buyers can be assured that the TSB was completed.