Someone just asked this on Facebook today. I've owned 3 aircraft here in Texas and have never received anything from the controller on them however I read a case on the controller's website (I have it scanned somewhere) where a Texas pilot claimed he had bought a kit so it wasn't an airplane so he didn't have to pay taxes. They wanted to rake him through the coals anyway.
He then came back and said : if you're going to tax it as an airplane then as a CFI I don't have to pay the taxes. Good try. (BTW that's how I didn't have to pay for my plane) BUT the controller told him he was correct, let me see your Sales and Use Tax certificate !
When you read the law it says you have to be in possession of a Sales and Use Tax certificate when you buy the airplane, and he got it later. So he ended up paying taxes, plus penalties, plus the attorney he hired for the years it took to get this all settled.
Having said all of this, laws change regularly so I'd check with a good CPA, Attorney, or if you write to the Controller's office, make sure you get everything in writing and be sure you are specific in your questions ! Asking if a CFI doesn't have to pay taxes will get you a "no a CFI does not have to pay taxes", but if you don't get the Sales and Use certificate then that could turn into "Yes you have to pay as a CFI because you didn't have a S&U Tax certificate when you bought it.