I store my nic in the 4oz bottles. Originally I filled these almost to the top. When I take one out of the freezer to use, I let it come to room temp after loosening the cap. As it warms, and expands back to the original fill point the pressure can be released. From now on I will probably leave a little more space, but still crack the cap to allow the pressure to escape.
Thank you as well oplholik.