I did both sets of my impressions at the Beltone Hearing aid place ($15) with neutral jaw position, slight open, jaw hanging a little forward.
The first impressions were done that way with my front teeth about 3-4mm apart and the LW were too small. But the second set the front teeth were about 20mm apart (more open) and the second set of LW fit better.
As she told me. she does singers with wide open mouths. (um, i'm still working this out with my perverted imagination)
My fit is exceptional. Wish I would have gotten my livewires a long time ago. I also wish I got their better customer service, I really haven't been impressed with their service at all. To me, its been like dealing with a big business, not a small, service orientated boutique.