Yes, there is--we take all of our snapshots from a slave, and we stop the slave before taking a snapshot, then XFS-freeze all drives, then take the snapshot, to ensure it's consistent.
1) The file system correctly orders or journals operations (I'm not familiar with XFS, but this is the case with FFS2/FreeBSD, ZFS, ext3/4 journaling, etc).
2) The database system correctly orders or journals operations, and properly fsync(s) to disk (which postgreSQL does)
Of course, there's no harm to an abundance of caution with something like this.