Is it safe to use DEFAULT currval()

I've got a table, and I'm trying to track changes to it, but can't do it via a
primary key including a revision-number (historical reasons).

CREATE TABLE foo (
  id       SERIAL,
  blah     text,
  tracking int4 DEFAULT currval('foo_id_seq')
  PRIMARY KEY (id)
);

I'm actually setting the DEFAULT on tracking using ALTER TABLE after the
event.

Now - it works, but is that a design feature or just luck?

It's convenient that tracking = id of the first entry, but not vital, so I can
always use a separate sequence if needs be.

--
  Richard Huxton
  Archonet Ltd