Toe in is all about lowering the amount of reflections from your side walls. If you aim your speakers straight out, the speaker (typical box-types) can fire pretty much directly to the left and right of themselves and you get more energy blasting the side walls… and reflecting back into the room and you.
These short-echo reflections (reverberation) damage imaging. Long-echos can actually enhance imaging. This is kinda how Magnepan and other dipoles can create such a large sound stage, one of the ear-brain phenomena knows as the Haas Effect. Some speaker companies figured this out and have rear-firing tweeters to enhance imaging. Now to get technical, short-echos, reverberation, can give a sound the feel of a larger room, but these short-echos also damage perception of instrument locality.
I learned about the Haas effect back when I purchased my first set of Magnepan speakers… this effect was pointed out as a major factor of why Magnepans image so well.
These short-echo reflections can also alter your perceptions of tonal balance too.
Anywho… speakers image best when placed along the long wall and not the short wall due to these short vs long reflections. I always try and put diffraction surfaces on the side walls to smear the reverb out even more.
Like everything audio, your overall room acoustics have a huge affect on tonal balance and imaging so what may work in one room, with a given set of speakers may not be the same with a different variable. And then there is the issue of taste. Experimentation is in order… no set formula.
Peace
Bruce in Philly