There is no such thing as a generally recommended best overclock. Nor a safest overclock.
The best overclock depends on your card and what it can do. Every card is different. Experiment.
No one said it is safe to overclock by any amount. Higher frequency creates more performance, more electrical draw, and more heat. More heat causes things to break. You can monitor your core temperature and try to keep that cool, but monitoring your voltage regulator and memory temperature is difficult.