Air bag seats It's all about if you are willing to take the risk. Chances are the airbag will still deploy even with normal thread. But will it deploy fast enough? Since I work for an OEM in trim development I have been in many meetings concerning airbag deployment. Its not enough to use the correct machine and thread and have it documented. To meet MVSS the units must also be tested. If there are any changes made to the seat cover (thread, materials etc.) validation testing must be done again.
Chances are a seat you have sewn an airbag seam on will never need to deploy, and if it does or doesn't deploy maybe you wont be dragged into court and blamed for a failure. Worse yet, held responsible for the failure. It only takes one time, to me its not worth the money I would make repairing an airbag seam. I know it sounds like a lot of BS but if I were in an accident that required the airbag to deploy I would want it to work as it was intended.