ZFS snapshots - to use pg_start_backup() and pg_stop_backup() or no?

I was trying to figure out how to get the following syntax to work:

echo "select pg_start_backup('zfs_snapshot'); \\! zfs snapshot
zroot/zpgsql(at)test; \\ select pg_stop_backup();" | psql postgres

The above command successfully starts the backup and creates the snapshot
but then fails to stop the backup. I've tried various combinations of \ and
\\ here with different whitespace and I just can't seem to find a
combination that works. I don't understand the proper use of \\ (described
as the separator metacommand).

However, in my research, I noted that a bunch of people seem to just not
even bother with pg_start_backup/pg_stop_backup and I guess aren't that
worried about the crash recovery process if they need to perform a restore.
I also find the omission of the start/stop backup functions from the File
System Level Backup page:
http://www.postgresql.org/docs/9.2/static/backup-file.html

Is the pg_start_backup() and pg_stop_backup() even necessary?

It would be nice to understand the proper syntax for the psql pipe,
regardless whether or not it is even necessary in this case.

Thanks,

Greg Haase