Unless you have real problems with inadequate suspension travel when the car is loaded up and towing, why bother? If you really do have such problems, I suspect that either the car is overladen or the springs have sagged for some reason. In exceptional circumstances, if neither of these two reasons apply, a set of adjustable shock absorbers might be a worthwhile investment (better than harder springs because they avoid a harsher ride when driving solo).
When the car is fully laden and the caravan hitched up, the centre of the towball must be 350 to 420mm from the ground. Anything below that figure shows that something is amiss somewhere.