Is listerine bad for my septic system?

Insignificant: In the total amounts used. Soap and detergents are worse as they clog the openings for drainage.

No: I presume you mean is it damaging to the bacteria that break down the waste in the septic tank. No, not unless you are spilling several gallons of it down the drain each day. The caffeine and other chemicals you pour down it on a regular basis have a much greater risk of causing any problems.