Short circuit operations in plpgsql?

I have some triggers. They need to do stuff when a new row is inserted
or when the value of a particular field changes.

So, something like:

  IF TG_OP='INSERT' OR NEW.foo != OLD.foo THEN
    -- do stuff
  END IF;
  RETURN NEW;

Is there anything that guarantees that the NEW.foo != OLD.foo won't
be evaluated (and hence fail, as there is no OLD) on an insert?

Cheers,
  Steve