Alot of people run truenas or freenas as a vm with or without the storage passed directly though and preset the storage as iscsi or nfs. Considering if you want to restart the virtualized nas VM, how important would you say unmounting the luns are first before hand. Alot of people seem to configure the VM to shutdown and start with the host and perform a rescan on start. Its nothing you'd do in production just curious what we think.
When ever I hear that some one uses FreeNAS for datastores my first idea is : oh dear - that can end ugly.
So I would highly recommend to be as cautious as possible - dont use shortcuts just to save a few minutes.