Short answer: You don't have to be exactly in the middle.
A 'what-every-audiophile-should-consider' answer: Very few people are particularly aware of their shoes as they enjoy a walk in the park or about town. Try sometime walking about in mis-matched shoes. Iin some mixed pairs one can walk well enough and enjoy the scenery in them, yet the journey is altered by an ongoing awareness of shoes being down there. It's an unnatural walking experience. At greater extremes (say, sneaker + cowboy boot), it's fatiguing and some aspects of the journey will be missed.
It's preferable to be relatively centered. Many of the audio cues superimposed on a recording by a playback room tell your brain that you are not really in the venue of the recording. The brain can sometimes overcome this and adjust to the space, by putting attention on the recording, etc. The fewer lop-sided cues you get, the more likely you will be to accept and ignore the playback room, and believe the recording.
Certain playback room cues (ones sufficienly delayed, and w/a spectrum similar to the recording) actually help to overcome some of the shortcomings of playback formats, and allow the brain to merge a bit of 'real' space with the recorded one, making playback more believable.
How detrimental it is to setup off-center depends on speakers' off-axis frequency response, listener distance from speakers versus walls, whether first reflections are absorbed or not, and strength of secondary reflections and reverb/decay time.
Re: suckout - If you experience a bass 'suckout' that improves when you move a few feet in any direction, its
not because a door or window is open. Bass holes can usually be traced to a modal or other acoustic problem, and these problems are most apparent when a room is sealed-up tight. That's why studios have to worry about room dimensions - because small rooms sealed tight for soundproofing reasons will be more prone to modal issues.
I knew I guy who swore that there was a suckout, because of a pronounced response dip no matter where measured in his room . . . I asked him if he had ever changed the
height of the microphone. oops. Everywhere he had placed his head and microphone was in the null between floor and ceiling.
All other things being equal, almost any rooms' bass
quality is improved by openings, esp if opening is large and/or near a corner. It functions somewhat like a bass trap does. However, a 'real' opening can zap bass much lower than any practical-sized trap could.
Many listeners are surprised to discover that midbass mud, etc. can mask the low bass - the range where the ear is less sensitive. Once they experience
clean bass in a room done right, the available quantity is
plenty. As always, your mileage may vary
.
-- Mark