Speakers "Disappearing"

It is dependent on the recording. But first, for the music to disappear, you really have to take the pains to find the best spot for them. Then you have to use a tape measure and make sure that they are equi-distant from back and side walls. Try to get them within 1/8 of an inch precision when you measure from walls. It takes days sometimes. But you will be rewarded.
Even if the music is intentionally panned, on a properly recorded music, it would be "free from the speakers" and appear behind and to the side of the speaker. But there are a few not-as-nicely-recorded discs, where the instrument/music refuses to leave the speaker. So, it is completely dependent on the particular recording.

@rlb61 ,
You are welcome. It is fun when the loudspeakers disappear. I am like "a kid in a candy shop" when this happens.

@kosst_amojan,
To each his own. I like the symmetry. Fortunately or unfortunately in my system, even a 1/2 inch difference is audible. Hence I make sure that I place the speakers the way I want.
Maybe it is not absolutely required, but hey, when you have the ability and affinity to do it, why not dial in to get the best out of your system.