There is rarely any reason to oc a video board. In one cenario, your machine is ancient, and you have no cash. OC the card, and hope it survives. In the other, you have a $5,000 PC and nothing you buy for it will make it faster. Time to push that envelope.
If you don't fit either of these, and you OC, you're just going to get a few frames and put yourself at risk of a major outage. I don't know about you guys, but my machine has no spare parts laying around.