There are no generally applicable rules on any aspect of speaker placement, including toe-in; speaker design, room acoustics, listening preferences matter. Toe-in results in less sound first bouncing off the side wall before reaching the listener. This tends to make the center image seem more sharply focused and prominent. However, this does tend to make the soundstage seem more narrow. Toe-in choices tend to be matters of picking the right tradeoffs.
Very severe toe-in, with the speaker axis crossed well in front of the center listener’s position, tends to make for a wider stereo sweet spot. This is the case because, for example, a listener sitting to the left of center will be closer to the left speaker, which will make that speaker more prominent partly because the sound arrives first (we locate sound sources using timing differences between sound reaching the left and right ear.. But, with severe toe-in, the right speaker will be more directly aimed at this listener so its higher volume partially compensates for the timing cue favoring the left speaker.