People are using 87 octane and no one's really announced any troubles. Though the car can run on 87 it will have worse fuel economy and you'll be down on power. Remember the TSX has a high compression motor just like the S2000 (although not as high a compression). It's more important to put 91 in the S2000 but I just wouldn't recommend putting 87 in a TSX.
Many owners of 09 tsx's complained of pinging problems even when using 91 octane or higher. I Personally have used regular gas on my 06 tsx with no problems. Excellent gas mileage with more than enough power for the driving I do. 140,000+problem free miles on a 5 1/2 year old car. Still with the original brakes. Will be replacing 2nd set of tires That have almost 70,000 miles on them.