One thing I should have pointed out (which, I think, is mentioned in the thread I linked) is that people vary in the amount of change of shape of their ear canal between different jaw positions. So for some singers (who use their CIEMs with their jaw in all positions, whilst performing), they retain a good seal, but other singers may lose seal during singing because their individual anatomy happens to be that their ear canal changes shape quite a lot as they move their jaw.
So, in that respect, it can be pot-luck as to whether your ear canal changes shape a lot, between different jaw positions.
As for tensing your jaw muscles, I would still avoid this as much as possible whilst having ear impressions taken, since you're not going to be tensing your jaw whilst listening to your eventual CIEMs, unless maybe you like chewing gum!
Wait and see. If you need to have a re-fit, then you'll be in good company - a surprisingly large proportion of CIEM buyers do not have a successful seal first time around (although I have found UM lab to be particularly excellent in their fit - they absolutely nailed my fit, first time, unlike FS, who were disastrously poor)
