"Isn't sodium benzoate said to be a potential carcinogen?"

I am not sure - it is listed as generally recognized as safe (GRAS) by the FDA.

I can't find anything in PubMed which suggests that there is any evidence that it is a carcinogen.

However, I take what I think might be your general point - when I brush my teeth I (as mrsD suggested above) just rinse my mouth with water rather than with Listerine or any other commercial product.