Like smeezekitty said, overclocking RAM can cause instability pretty quickly.
That being said, as long as you don't increase voltage past what the ram and the memory controller can handle, then overclocking it is personally safe. I've got some (sadly now discontinued) 'miracle DIMMs' from Samsung. They were $54 for 8GB of DDR3, 1600, Cas 11 sticks that ran at 1.35v. Running them at 1.5v, I'm hitting 1866, Cas 7... and that's not even pushing them as far as they can go.