Walang set rule. Kanya kanya. For us here at TRACKStudios, we point the convergence points of the 2 HF drivers about 1 or 2 feet at the back of the head of the mix engineers. There is no stopping you from aiming the speakers straight towards the face of the mixer or towards anywhere, as long as you get a good mix doing that (you gotta have a reason, though, for doing that). For us, aiming them towards the back of the head gives us a better stereo spread.
FWIW