The speakers and I sit in an equilateral triangle nine feet on each side, and they point directly at my ears.
A lot of good replies to your post already by some knowledgeable folks, but the reality is there is no etched in stone rule. Dispersal pattern of all drivers, room characteristics and your own ear height in the seated position all matter. That said, and if I'm not mistaken, I do have reason to believe that speakers are best towed at the listener when they are *NOT* aimed *directly* at your ears, but instead *directly behind* your head at ear level. Subtle, but meaningful.