In this case, I think the legality is secondary, doing the right thing is primary. And having mud flaps on a commercial size vehicle is the prudent thing to do, even if its not required by law. Of course it varies by state. As far as the Texas reference, I would think that law was written decades ago, before super singles came along, even on trailers. If you were pulling a semi trailer in Texas with super singles and no mud flaps and got pulled over, I seriously doubt that you would be able to get away with telling the LEO that you didn't need mud flaps because of the singles.