However, there's a way to prevent sieged tanks from shooting. This is useful to have a volley ready against any templar emerging from a shuttle, and to prevent tanks from shooting at zealots in their midst while vultures clean up.
If a sieged tank's cannon is facing its target, then you can't stop it from shooting. However, if the tank's cannon is not facing its target, then you can spam the stop command to severely slow down how quickly it will rotate to face its target.
So, to have a backup squad of tanks ready to shoot emerging templar, you can target a mine or vulture in the back of your army, away from where the Protoss is coming from, and then spam S.
Furthermore, you can even target the unit in the back, then press S before the tanks align their cannons against it, so that you don't even have to shoot at your own unit. Then, you just spam S until you see a templar. As soon as you see it, you click on it.
You can also select a group of vultures and pre emptively spam right click under the shuttle. This will make your vultures move under it, but as soon as a templar comes out, you will click on it and one shot it (you can also just move them in general and reactively click on a templar). The vulture projectile will hit the templar faster than he can storm, if the vultures are close enough.
In some situations, you can even throw pre emptive EMPs at the shuttle to deny storm.