Risk Free? No, there is a risk. Very low risk, but its not risk free. Depends how careful you are.
As far as how much you're gonna get, overclocking is just luck, you might not even get 5mhz. Cards are just guaranteed to run at the stock speeds. Its just luck.
As far as its "worth it"... thats a personal decision. You could damage you card. Or you can actually get it running faster than it should, without any negative effects. Its worth it to me. I'm very careful with overclocking to be sure I don't damage anything.