everyone that buys a knife tests to see if the knife saisfies them, i think it's safe to say everyone here tests their knife for sharpness on regular basis, when you bought it you probably tested various grips or if you can move the clip elsewhere & so on, testing the lock is a legitimate thing to do because as much as i trust spyderco, i will not blindly trust any knife (or any other potentially dangerous object for that matter) without making sure it's safe, not even a fixed blade.
so the question is, like Sal said, when does testing cross the line to abuse?
spinewhacking is a reasonable test, while you work, it's quite possible the knife recieves a light whack in it's spine, it happened me & probably to many others, so no reason not to test for it but first, several things must be taken into account:
what is the knife designed to do? for a delica it's enough to survive a light whack against my palm but obviously it's not enough for a chinook.
what would happen if the lock failed? those of you that have natives, take them for a minute, hold them with the finger in choil & then disengage the lock - the blade can't reach the fingers because of the choil.. i even took this one step further, i disengaged the lock when my finger was in the frn choil & let the blade drop - again, the blade choil prevented any dmg to my fingers. so in the case of the native for example, a lock failure is most likely to cause you no injury so in this particular knife there's no reason to test the lock at all. on the Centofante III on the other hand there's no choil & no kick, if the lock failed you would be injured, however, it's a genteman's folder so it should see no use capable of causing lock failure & light pressure with your thumb while the blade is open is probably all the test you need.
to sum up, it's a legitimate thing to test your lock, but excersizing common sense in testing will prevent both damage to your knife & injury to you.