Acoustic wavelengths vary from 17m for bass frequencies to 17mm for the very high end. So phase is absolutely an issue. If you want to hear this for yourself,

Acoustic wavelengths vary from 17m for bass frequencies to 17mm for the very high end. So phase is absolutely an issue. If you want to hear this for yourself, play sines wave between 1k and 10k (if you can hear that high) through some stereo speakers. Move your head and you will hear some very clear diffraction peaks and troughs. You only really lose stereo location below around 200Hz, which is around 150cm. With domestic speakers, room reflections and other influences make it hard to hear phase issues. But in a studio you want to hear what's happening as clearly as possible. Some brands - like ATC - put quite a bit of effort into keeping phase coherency.