To expand on Earle's post: I was fortunate enough to discuss this very issue with Joe Kines at an alumni event.
Kines said the timing of the look back, and whether to do so at all, should depend on the defensive position of the DB.
The phrase Kines used was, "in phase." He said that if the DB is in phase with the WR, he should look back when the receiver does. If he's not, even if it's by only a half- or even quarter-step, the DB shouldn't look back, but should try to disrupt the catch. Failing that, he's still in position to make the tackle after the catch.
Reason being, if the DB isn't in phase, and looks back anyway, the distance between himself and the WR invariably increases. In other words, the DB exacerbates the problem.
Compounding that, if he's the last line of defense -- i.e., there's no deep help behind him -- you risk turning a 15 yard gainer into a TD.
The definition of "in phase" was more complicated than I ever would have thought. It had to do not only with being on the receiver, but also figured in the DB's hip and shoulder position relative to the WR and other stuff that was honestly lost on me.
But for the average fan, of you think of "in phase" as "having really good coverage," you'll have the concept.
Kines also said this was about the hardest technical thing to teach the freshmen. To that point in their athletic careers, they have always been head-and-shoulders the best player on the field. They've been able to make up for suboptimal technique with sheer ability and speed.
So they have a really hard time admitting, even to themselves, that the WR got the better of them, even if by only a hair.
Unfortunately for them, SEC receivers eat that for breakfast. Some learn after being burned a couple of times in practice. Others have to get their pants yanked down in a game. Still others never really do.
What I took away from Kines' explanation was that whether and when to look back is a lot more complicated than many of us ever thought.