To address an earlier answer that has since been deleted (together with my disagreement comment)... Some people suggested that "I do not recall" is an example of Taking the Fifth.
But we respectfully disagree. In the "I do not recall" statement, one is alleging that one has no recollection of a certain event or facts, which is clearly incriminating in case it could be proven that one, in fact, did have such recollection, or must have had. It is also clear that it's entirely different from maintaining silence or explicitly saying only enough to the effect of doing so.
Of course, at first sight, proving someone's faulty or perfect memory is supposedly less straightforward than an outright denial of fact, but it's not any less of a lie than an outright denial, so, as another answer by chapka points out, it's not that difficult to introduce the evidence to the contrary of the "I do not recall" statements (videotapes and a thank-you-for-your-bribe notes), and, I must point out, if you must defend against such evidence knowing full well that your original "I do not recall" has been false anyways, you might as well be committing another perjury.
As to whether it's useful or not generally? If you must answer a bunch of questions, and only certain answers are prefaced with, "I do not recall", then it arguably might have some effect of what you do and don't remember. However, if you preface every single answer with such a statement, then it starts being somewhat of a farce.