A qtree does not hurt you and if you ever plan on a async replication like qtree snapmirror and/or snapvault, you do need qtrees.
You can add qtress to existing volumes and online move luns into them if its within the same volume btw:
filer> qtree create /vol/san/disk1
filer> lun move /vol/san/disk1.lun /vol/san/disk1/disk1.lun