Enabling XMP usually fixes the frequency and timings for you but sometimes you need to set them manually. I would go with enabling XMP in the BIOS and if that doesn't set things properly, manually set it.
You should set it to the advertised speed.
This depends on a few things.
1 what your doing with the PC for gaming when you get to 1600/ 1866 mhz that is fine running memory at 2400 or something will show no performance increase.
2 what speed it's at.
3 how much of a bump in volts will be needed to run OC and be stable.