If I wanted to push 6400MT/s, which timings are normally the ones you would loosen initially?

Was able to run a CL30 6000MT/s at a tighter 6400MT/s. This set is running a lot tighter than fist set, but struggling getting it past 6000, even with 1.5v+. Was wondering if there was a usual set of suspects to loosen before the others.