Here's the thing: The shape of your ear canal changes when you move your jaw. Thing is, there is no way predict whether it gets bigger or smaller. In fact, with some people, the left and right ears will behave differently! So the use of a bite block may help, or it may screw things up. This is not an exact science.
Remember IEMs were originally created for musicians. Most all of them - but especially vocalists - are changing their facial expressions throughout a show. When the seal is lost, bass response is seriously compromised through the leak.
It's my theory (and only a theory) that the bite block idea originated as a way to maintain seal during vocals. And it obviously helped enough people that it has become recommended practice with some (but not all) manufacturers. OTOH, home listeners are much more stable while listening. A good fit should be obtainable without a bite block, especially if you listen with your mouth closed.
This is the part where I sing the praises of silicone earpieces, like all Sensaphonics models. They flex with your canal, maintaining seal through a wide range of motion. They are also soft, and thus more comfortable.
Sensaphonics does not recommend the use of a bite block when getting impressions. Quoting from the company's "Getting Ear Impressions" PDF:
When making ear impressions, start with your jaw open. Once the material has been
injected, move your jaw to replicate the facial movements you make while performing.
While the material is setting, vocalists should sing, horn players should bring a
mouthpiece and play, etc.
Obviously, this procedure is designed for musicians, and our percentage of refits is quite low. My point here is that pointing to bite blocks, swallowing, etc., as causes for bad fit oversimplifies the issue. Again, this is not an exact science, and what works for one person may well create problems for another. This is the whole reason that custom manufacturers offer refits.
Edited by JackKontney - 4/22/11 at 1:03pm