PostgreSQL 9.1: vacuumdb vs. autovacuum

Question

Have a question on the difference between just allowing autovacuum to take care of vacuum'ing or having to run a full db vacuum in a certain interval (every night?, every week?).

We used to run a full vacuumdb every night before a full backup. It used to take around an hour and a half. Then we stopped and just allowed autovacuum to do it's thing.

Is that a good idea? Should there still be a vacuumdb run every night, week?

I noticed some processes tonight...

"autovacuum: VACUUM public.large_table (to prevent wraparound)"

Is that normal or is that because we stopped running a full vacuumdb every night?

Thanks all for any insight.