I can't recall if sodium benzoate was in every bottle of lemon juice that I used, but if I remember correctly, it's just a preservative. I did however use lemon juice for my first grow. I used several bottles actually. It worked pretty well, but the cost started adding up fast since I had to use it every time I watered/fed nutes. So I made the switch to using General Hydroponics PH down powder.
You can also run into a problem with the lemon juice in the soil attracting bugs, or worst case scenario, attracting fungus gnats, which you absolutely do not want in your grow.