I've noticed that, due to the majority of my VMs being in PoolY, which is now very far down a list of things that were created by FreeNAS like:

PoolX

PoolX/iocage/a

PoolX/iocage/b

.

.

PoolX/iocage/j

And then comes PoolY

More than likely, once you start creating VMs, most people are going to keep putting them in the same location until things fillup and/or more storage is added. It's probably a good idea to keep the 'last used' as the default for the location when the wizard wants to put in a zvol location.