reliable backup techniques

I'd like to get some opinions on the best techniques for getting
reliably consistent backups.  My current understanding is that the
only reliable way to get a consistent backup of a running system is
through pg_dump or pg_dumpall.  More specifically, it is my
understanding that file backups (tar, etc) do not guarantee
consistency due to the potential lag in cache writes to disk from
running transactions.

Is my understanding correct?  Are there other ways of getting
consistent backups of a live system?

Thanks,
Ed