This subject is on my mind right now. For years, I was listening in my room with no toe-in. About two years ago, i decided to toe in my speakers to see what, if anything, i might be missing. At the time, the toe-in I achieved allowed me to 'improve' the width of my soundstage and I was happy with that. Now about three months ago, i decided I wanted to hear what my old set up sounded like, so back to zero toe-in I went. Well to say the improvement is quite noticeable and in all ways better would be an understatement! I had noticed a very slight amount of brightness when the speakers were toed-in, assuming that this was part of a source problem, as the issue came and went depending on the recording. Now, that issue is no more, the horns again have the right amount of blat, the depth and soundstage is even wider and the bass is simply more there. No issues with any type of fatigue, that I now put down to the tweeters aimed directly at my ears before. So, although it clearly can be beneficial to toe-in, and highly system room dependent, the lesson here, at least for me, is that this not always the case.
From now on, my speakers are straight ahead.