Everything in relation to OC is safe until and and unless limitation are maintained. Assuming that you have downloaded NVIDIA nTune Version: 5.00.06.06, which is the latest, I am giving out a .pdf file which would help you get better accustomed with its use.
here is the link download the pdf for instructions:
*rapidshare.de/files/24559400/NVnTune50_UGv11.pdf.html
Regards,
Shovik.