I have a Evga GTX 580 SC and i was testing how hot it could get i was also testing the stabilty of the GPU
the GPU Maxed at 98c then fan kicked in hardcore and cooled it off real fast
now will doing this hurt me card? only did it to see if my GPU is what is said it was able handle on load
and to test if MAX temp it would reach before safety features of GPU turn on
Can it hurt your GPU? Yes. Will it? Who can tell? All chips are made on an assembly line, therefore some are stronger than others. You may have the one that was soldered badly and it may fail early.
You also may have the superman of all chips, that will never die no matter how badly you abuse it.
My advice? Limit your tests. Unless you're ready to upgrade your GPU, I don't suggest pushing it to the very limits.