As iamagloworm notes, Sensaphonics does not recommend using a bite block. And yes, we are Shure's recommended manufacturer of custom sleeves for their IEMs.
Sensaphonics has been in the business of manufacturing custom IEMs since the mid-90s, and custom earplugs long before that. Dr. Michael Santucci (founder, president, and a practicing audiologist) has developed his recommended procedure for taking impressions over the course of a lengthy career working with very demanding musicians. He actually trains other audiologists in working with musicians and taking accurate impressions, as this sort of elective equipment use (CIEMs) is not taught in university audiology curricula.
He notes that, while using a bite block when taking impression has become popular, there is no clinical evidence that using one - or not - will produce more accurate impressions. I should also point out that the earpiece material (acrylic vs silicone) is NOT a factor. (Manufacturing technique, OTOH, is a huge factor in creating properly fitting custom IEMs.)
Factually speaking, the shape of the ear canal does change with jaw/facial movement. But the WAY in which it changes (direction, degree, shape) is truly individual and not predictable. As the jaw opens, some canals get wider; others get narrower. And by varying amounts. Maddening, but clinically verifiable.
So there is no universal solution. And anyone who tells you differently is, IMHO, selling snake oil. Sorry.
Edited by JackKontney - 4/1/13 at 2:08pm