It has been my experience that "topping" mature trees usually makes more problems than it solves. If the trees are truly causing a problem with your septic lines, your best option is to have them removed. This is a heart-wrenching decision to make, and I would only recommend it if you can prove that the trees are indeed causing problems (backed up sewage, etc.). Otherwise, I would recommend a Wait and See period. Topping will not cause the roots to "retreat"; it may stimulate more root growth. Alternatively, can you move the septic lines?
More importantly, a topped tree often looks bad; topping destroys the natural shape/habit of the tree. In some cases, it eventually leads to slow and ugly death.
I have a row of hybrid poplars only about 20-30 feet from my entire septic system (lines, tank, leach field). These are 60 foot trees, 25 years old, and water hungry. Despite dire predictions, I have had NO problem with root invasion. The tank is pumped (by law) every 3 years.
Perhaps your local town officer could help with info before you start ruin the trees appearance and believe me, once you cut a mature tree,at the top, it will be an eye-sore for the rest of the neighbourhood and your outlook too.
As a guide, most tree's have a root spread to equal the width of the spread of the top, that's the branches / foliage ect, so if the branches spread out for say 15 / 20 feet then the roots will be spreading that amount too, the thick roots that stablise the tree as it grows may still be 2/4 that but the other roots that go out searching for water / moisture are more likely to cause the damage at first before the larger roots take up the moisture (septic tanks, water /drains and any other water features on your property.
Sadly we never think about all that when we plant tree's, we move away and someone else has the problems to deal with later years away, I would also do as purpleinopp has suggested and consult a REPUTABLE tree surgeon who knows exactly how tall, wide and damage caused or NOT, by all /each individual species of tree, this first consultation with tree surgeon should be free until you wish him to carry out any work for you.
Good luck, don't be rushed but at the same time, don't wait too long before any damage is beyond just felling a few tree's.
Good luck, WeeNel.