Not sure why you think it will get damaged by being benchmarked. Maybe if you do some crazy overclock. But it should be able to run FurMark all day everyday, for years.
The card is simply doing what it is made to do, why do you think you would hurt it? If you do run into a problem, it is because the card is defective, in which case you should return it for replacement!
There is non-destructive testing and destructive testing. As long as you remain within the max rated specs - NDT - you can go till the cows come home(or longer) Destructive testing on the other hand is used to find the "breaking point" - I suggest you don't go there.
-Bruce