I just leave mine on Stock. It already needs high fans on some games (Guild Wars 2, Far Cry 3) so overclocking it for a measly few frames will do more harm than good.
It's not like it doesn't play games on high / ultra quality with good frames anyway.
Running my EVGA GTX670 on stock, I generally don't tinker with my GPU out of fear of borking it
I have an EVGA GTX650 Ti SSC
and have overclocked it a fair bit, even past the good clock it comes with
EVGA honor all warranty, even if the card is clocked (with EVGA Precision)
They are one of the only manufacturers I know that do