Generally when you go over a bump the car should settle straight away. If you push down on the car and let go, it should come back up to height and then settle. If it dosn't then your shocks are getting tired.
My cars not much over 60k and it's starting to feel a little loose. For the price of new OEM Sachs shocks from Euro Car parts (when they have a discount on), i think i'll change them out this spring along with top mounts and the WALK kit i have.
If you are changing the springs out and your car has done a decent milage there is nothing wrong with a new set of shocks, too.