I had my new ear moulds done before Christmas. I have often wondered about how the ear canal changes when you smile. So I asked the audiologist if I should smile when the moulds were being made. She said "no, but there is a specific thing that you can use to keep your mouth open while the stuff sets in your ears". This makes the seal of the moulds better. This is my 3rd set of moulds and easily the best yet. The seal is excellent. Do it.
Just make sure you have a tissue cos things can get a little drippy from your mouth after you have kept it open and immobile with a wedge between your teeth for 2 minutes +.