Ive been running my GTX 1080 for over a year with +20% power/+100/+1000 mining equihash. It mines a good 20 hours out of the day, I only shut it of in the evenings if I am on the computer. So far I have seen no issues whatsoever with running it this hard. I also have 1070s running like this for over a year as well so I wouldnt think you would cause any damage as long as you keep temperatures in check.
I know when you set at +20% the card becomes hot and you'll smell something like melting plastic or rubber, especially if you're mining Ethereum. So, if I overclock all my graphics cards will they not be damaged? Well, you know graphics cards nowadays are too expensive and hard to find.
Also, may I know the required or safe temperature (°C) for the CPU and graphics card?