I'm reading The Sounds of Japanese (Vance 2008), which is an introductory textbook in Japanese articulatory phonetics. The first chapter lays out some basic concepts in phonetics, and although the book is essentially about Japanese, the author gives some English examples to illustrate those concepts (the variety described being "United States newscaster English", according to the preface).
Vance uses the English [ʃ] and [ʒ] as examples in a section on secondary articulation (p.20):
Another common secondary articulation is labialization, which is simply lip rounding superimposed on a consonant articulation. The English lamino-postalveolar fricatives [ʃ] and [ʒ] are usually labialized, as you can see just by watching a native speaker pronounce words like dish and beige. The IPA symbol for labialization is a small raised w after the appropriate consonant symbol, so we could transcribe dish and beige very narrowly as [dɪʃʷ] and [be͜iʒʷ].
And sure enough, when I asked native speakers around me (all Californian) to pronounce these words, I saw (very obvious) lip rounding. But I'm from Illinois, and I don't think I round my lips in dish or beige. This got me curious, so I asked other speakers online, most of whom were from Northern Illinois or Wisconsin, and none of them rounded their lips, either. (Two took videos, but I'm afraid I can't share them here.)
I'd like to talk about the differences the between Japanese [ɕ] and [ʑ] and the English [ʃ] and [ʒ], and one of the textbook differences seems to be this labialization. I'd like to know how widespread it actually is so I can write an accurate description. But I'm afraid my poll wasn't very scientific, so I'm not sure I can draw any conclusions from it.
I have one textbook on English phonetics, A Course in Phonetics, 6th ed. (Ladefoged & Johnson 2011). On page 67, the authors write:
The English fricatives /ʃ, ʒ/ are strongly labialized, and the fricatives /s, z/ are slightly labialized.
From this description it seems like it should be a universal feature, but it doesn't seem to be.
How widespread or common is labialization of [ʃ] and [ʒ] in English dish and beige?