well manual overclocking is always safer if u know what u are doing.
I'd just do it manually.
And yes overclocking too much can do some damage to the card, though it's very rare. I don't have any experience with that kind of programs, since I've always done it manually. But usually the damage to the card is done by either changing the voltage or the memory thingys. (don't remember what they were, could have been timings
) so just changing the clock speeds is quite safe. But still, keep an eye on the temps.