a modern system should not really need it (the evaporators have an anti-bacterial coating), but if you like it, use it, it can improve the general smell of the car, a bit like febreze. First though is to check the pollen filter and put in a genuine one if it looks cruddy, not a pattern one. Check the heater inlets for leaves and general crap and if you can find them, check that the heater drains are clear (generally a short tube with a flattened end that lets out water and keeps spiders from entering, these can block with leaves and allow water to accumulate inside and go stagnant.)