PITR base backup -- stop server or not?

The docs for Making a Base Backup (tar) say that it can be done live
without stopping the server:
http://www.postgresql.org/docs/8.3/interactive/continuous-archiving.html#BACKUP-BASE-BACKUP
(step #3)

However, the docs for straight File System Level Backup (tar) say the
server must be shut down:
http://www.postgresql.org/docs/8.3/interactive/backup-file.html
(restriction #1)

Is this because replaying the WAL files will fix any of the issues
listed in the File System Level Backup restriction #1?

Thanks,
Rob Adams