Is it necessary to have your cesspool/septic system serviced regularly?

We've asked around, but haven't found anyone, so far, who has their tanks drained. However, all of us use Cess-Flo, or a similar product, as often as is recommended on the package. Is this adequate? A few other things you may need to know: we have two tanks, we are a family of three, and we don't use excessive amounts of water. We recently had a "major plumbing service company" do an unrelated repair and then try to convince us that it's necessary to service the septic system every 2-4 years. We are just wondering if it's true, or if they were simply trying to sell us another service. Thanks for your help!