This will be an interesting debate... if you plan on using SnapVault or Qtree SnapMirror then creating a in a qtree makes sense (although you can replicate non-qtree data with "-"). If you are creating one lun per volume, it really won't matter... if you are creating multiple luns in a volume, I like creating a qtree for each server so that there is still a logical name separation between luns for different servers.
Those who have been working with ONTAP for years will likely vote for a qtree and newer will vote for in the volume. Both are acceptible and I of of the former bias on it and tend to create qtrees. Down the road, we may see qtrees go away (I'll leave it at that) so any replication based on qtree would be a consideration but that is some time from now and wouldn't affect using it as a container today. Also, you can move luns by renaming within the volume to put in a qtree or in the volume without affect. Here is a kb on how to move to a qtree or out, so you can change your mind later is the good news. https://kb.netapp.com/support/index?page=content&id=1012082
