Authentication to run pg_dump automatically

I would like to write a script to dump all the databases each night. The
only way I have figured out it can be done is to trust the 'postgres'
user in pg_hba.conf for local connections and run pg_dump with that user
in the script (is this safer than PGPASSWORD). I am looking for advice
on this, does the postgres user have privileges that are not necessary
to do a pg_dump. I want to be sure I trust a user with minimal
permissions, but still be able to dump all. Who has experience with the
best way to handle this?

This is for a PostgreSQL v7.1 database, so I can't use .pgpass and I
don't want to use the environment variable PGPASSWORD.

--
Robert