BBQ iginter power surge
The capacitors should go across the power supply terminals of the various components like the shift register and igniter, to bypass noise riding on top of the power. When noise comes from some other device, the capacitor can help shunt it to ground. Reciprocally, when a sudden, brief current demand comes from the local device near that capacitor, the capacitor supplies the demand, so that there is less of an effect on distant parts of the circuit.

For better answers, you might want to show the exact circuit diagram, including power supply details.