The effect of NaCl on the ability of an acid to protonate benzoate
Some salts added obviously will alter the pH value of the soda ($\ce{NaHCO3}$, for example), and others not (like the example provided by you, $\ce{NaCl}$). Changing the ionic strength by addition of salts may render your mathematical model to describe the composition of the analyte more complex.

Hence, let me I suggest an alternative to outgassing the soda: immerse the open container into an ultrasound bath.  This works rapidly, however ensure the container is widely open and may accommodate the foam briefly generated.  Otherwise, it may surprise you as a sticky geyser.  Second hint: gently approach the active regions in the ultrasound bath to moderate the gas evolution.

(Degassing solvents with ultrasound is a common preparation for HPLC, too.)