The bump test shows if you have low speed dampening atleast.
I think initially you will see worse broken pavement performance out of a worn shock, so the tire will lose contact in a section of road patches. Its easiest to tell by going around a corner with some broken pavement and pot hole patches, and the car skitters sideways a little.
For me anyways, I wait until the shock is either making bad noises, or really won't keep the tire on the ground on rough pavement, or fails the bump test. Which means I don't replace struts very often.
1 rear strut went on my Neon at ~140k miles and the other one tested fine according to the brief bench test in the FSM. The Tracker has its original front struts as well, they are starting to skitter a bit, but more understeer isn't a bad thing with it.