This is a really interesting thread. I like the variety of views in it. I've been teaching tuning & intonation to an a cappella chorus of some 60 women, doing one on one, small groups and whole chorus sessions. It's a barbershop chorus where the goal is always to sing so accurately that the 4 parts create strong overtones, so strong that the audience will hear at least 5, maybe even 6-part harmony. (Side question: Do traditional, unaccompanied singing ensembles have that goal too?)

This chorus is where I've observed singers tuning differently depending if they mostly hear their inside-the-head voice or their outside-the-head voice. Basically, tuning to the inside (bone-conducted) sound caused them to sing a little flat. By which I mean anything between a hair and 2/3 of a semitone below the desired pitch. Usually the discrepancy has been larger in the lowest range and smaller in the highest range.

Cupping the ear is a wonderful tool that has helped a number of these singers learn to match their "outside" voice to the pitch pipe or the voices around them. I'm sure their inside voices are beautiful but since no one else can hear them, to focus on tuning them seems futile, even counterproductive.

In our case the ear-cupping is used only in rehearsal and personal practise, never in performance. The best way for us to use this method of tuning is to cup the ear constantly at first, then uncup for a few measures at a time, re-cupping to check if tuning is still accurate. The learner tries to go longer and longer uncupped without losing accuracy. Not surprisingly, the chorus was stuck in the competetive rankings until we got a critical mass of singers sufficiently aware of how to tune more accurately.

Because I've had this experience, I'm fascinated by the comment somewhere above (or was it in the linked thread...?) from one who found that unaccompanied singers usually go sharp. Even with the tension caused by performance nerves, I've almost never seen people go sharp except when over-compensating from fear of going flat.

A fascinating topic.

Oh yes, one other thing--in a chorus this size singers usually cannot hear their own voices (those in the back row or side margins excepted). Cupping is very useful to check what one's voice is doing, pitch-wise as well as vowel shaping & timbre, important because everyone matching these things is critical to get a chorus' chords to ring well with overtones.

~CH.