[GENERAL/INTERFACES] Dynamically detecting need to vacuum?

I'm seeing a few intermittent show-stopper errors (some of which I've asked
about previously in these forums) which go away after vacuum is run.  Pgsql
documentation recommends to vacuum nightly or after copying a large class
(not sure exactly what that means) or deleting a large number of records.
I'm tempted to run vaccum almost anytime I detect Pgsql errors (via perl
DBI) in order to try to salvage/retry the query at hand on the fly at the
cost of a much slower query from the user's perspective.

I'd like to learn how to programmatically calculate/detect when a vacuuming
is needed, and would like to hear any advice on how folks are successfully
doing this or think would best be done.

Any comments?

[Hackers:  it'd be nice to have pgsql optionally take care of this...]

I'm using Pg 6.5.2 on RH Linux 6.0 2.2.5-15smp via perl DBI/DBD::Pg...

Thanks in advance,
Ed Loehr