When vaccuum full should be performed with postgresql-8.1 and autovaccuum?

Hi all,

   I'm migrating our DDBB from postgresql 7.4 to postgresql 8.1. In
postgreql 7.4 we don't have autovaccum running. So, every night before
doing the daily backup we do a "vaccuumdb -f -z" for each DB. My doubt
is, the autovaccuum  with the version 8.1 is started by default and I
don't know how often I should perform the "vaccuumdb -f -z" or now it's
not necessary to do it at all.

Thank you very much
--
Arnau