i very rarely do unless they are clearly damaged or the rubber parts is cracked,, as if the rubber is then it will be weak and would allow the shock to over move go past the point it was designed to go it to and cause it damage maybe,, say if hit a pot hole hard sometime,, that said to do this the car would prob have bottom arm and wheel damage to go with it,, and maybe the extra up bumped stopped travel and stop could add to it,
i would also while doing that job replace the top strut mounts with new ones as a future proof exercise as while strut and spring off only do job once if that makes sense,,
this all said on all my own cars i'm fairly anal about these kinds of jobs and just do it once will full kits knowing it should be good for another 70k plus miles,, rather than doing one things to find something else is also getting worn out,, sometimes the saving a few quid will bite you on arse where a worn part will wear out the new part a lot sooner.. rubber on cars tends to have 7-10 year life before it gets soft and cracks as a rule,, and mileage has a small bearing on it oil heat cold water how much its gets hammered when car driven,
