From a subjective viewpoint, I'd suggest 'by ear' for many models with unknown dispersion characteristics.
Some nice speaker manfacturers do make suggestions for the best listening axis, for example from memory, my three way ATC's of old used the midrange driver axis and mid-tweeter drivers set on the inside of a stereo pair as best for setting height and toe in. 'BBC derivative' types seem to need the main tweeter axis and toed in to point at the listener. Popular PMC slim types seem to like the tweeter just above but the hf peak they introduce in means they balance best pointing straight ahead and the listener twenty or thirty degrees off axis.
The tests here can be an enormous help if one can learn to roughly interpret the dispersion graphs and in fact, I'd say it's the *vertical* dispersion and interference that could maybe be beneficially adjusted by tilting back or foward a little.
Of course, all my text above can be rendered null and void depending on the room you have and the reflections therein!!!
Just my thoughts...