Each state, and some cases even each county/city, sets their own regulations. And even change depending on what type of coupon it is. Some treat it like cash (so the tax is on the pre-tax amount), some don't and reduce the taxes. So if you are traveling, it is difficult to argue as you don't know the local rules.

I tip on the pre-discount amount - same as I do if I've received free food, etc... if I've sent back a meal IF the service and service recovery was otherwise good.