The per-query error-handling callback is rather straightforward. If the callback returns true, the entire transaction is rolled back. If the callback returns false, the transaction continues as if nothing had gone wrong.
Thus, if you are executing a query that is optional — if a failure of that particular query should not cause the transaction to fail—you should pass in a callback that returns false. If a failure of the query should cause the entire transaction to fail, you should pass in a callback that returns true.