While it may vary somewhat with speaker design (dispersion), I believe most speakers will benefit from placement at least 3' from the sidewalls, measured to the HF driver. From my reading, it is desirable to have direct sound arrive at least 10 ms ahead of reflected sound to reduce muddiness and smearing. Fortunately, sound travels at approximately one foot per ms so this makes for an easy calculation. Using a tape measure, the distance from the speaker baffle to the prime listening spot (PLS) should be at least 10' less than from the baffle to the first reflection point on the sidewall and then to the PLS. From this you will see that sidewall distances greater than 3' may be even better.
If you have not read Richard Hardesty's "The Audio Perfectionist" site, he believes most systems are set up with speakers too far apart and that some form of isosceles triangle set up is best, not an equilateral triangle as we once believed.
If you have not read Richard Hardesty's "The Audio Perfectionist" site, he believes most systems are set up with speakers too far apart and that some form of isosceles triangle set up is best, not an equilateral triangle as we once believed.