RHN maintenance/entitlements for machines not connected to RHN

We deliver workstations to a customer that are pre-loaded with RHEL 6 Workstation. These machines are on a private network and are not connected to RHN or a local satellite/mirror. When we first ordered the workstations we signed up for multi-year access to RHN. These machines are not routinely updated with new software from Red Hat. What I'm trying to determine is if the RHN subscriptions need to be carried if there is ever a chance we might need to update software on those machines.

Outside of that, the company I work for has many entitlements for the workstations used by its employees as well as a local RHN mirror. If the packages on the deployed workstations ever needed to be modified, we would have to burn RPMs to a DVD and ship it to the customer. Since the RPMs were indirectly obtained from RHN, I suspect a subscription would have to be in place prior to installing them, but I'm not sure. Can anyone provide any clarification on this situation?