I overclock both, but I'm pretty conservative. If an OC becomes unstable, I generally just back it off, rather than try all sorts of things to get it to be stable at that higher setting, especially with GPUs.
With CPUs, I'm very conservative with voltage...I do not want to be anywhere near the maximum safe voltage, preferring to be as close to stock voltage as possible. I can easily run my 2500K at 4.5GHz (and if I get a better cooler, I'm sure 4.6-4.8 would be possible), but I'm just fine at 4.3...it's barely over stock voltage for stock speeds, and I'm running cooler than with the retail Intel HSF at stock speeds, so I feel good.
My GPU, I will NOT add voltage to...I just OC to where it is rock solid and leave it there. With my GTX 460, I was stable at 940/2150 for a while, but I'd occasionally get a 'driver stopped responding' error after heavy gaming, so I backed it off to 925/2125 (I think the memory could still run higher, but I don't care much)...and now it's rock solid for hours on end. I am not worried either, since the EVGA warranty on my card is 3 years (not the lifetime on this card, unfortunately), but it covers overclocking too, so if I fry the card, they'll send me a new one (not that I want to or ever intend to push it far enough to fry). In 3 years I'll have a new GPU, so no worries.