It is simply wrong to state categorically that any particular degree of toe-in is optimal and anything else indicates some kind of problem. All adjustments--placement, toe-in, rake angle (tilt back) are specific to a particular speaker model, room acoustics, placement of the speaker and listener, listener taste, etc.
With most speakers and situations, the ideal degree of toe-in involves trade-offs. With little or no toe-in, the soundstage will tend to be wide and open sounding, but, the center image might not be as strong and pin-point tight. By increasing toe-in that center image will tend to get more pronounced, but, usually the trade-off is a woundstage that does not sound as expansive, or the listener does not feel quite as immersed in the soundfield. The "right" toe-in is where the listener is most satisfied with the balance of trade-offs.