As Roy Cooke would say: "It depends." There is no hard and fast rule but there are some guidelines to think about, all which must be viewed through the prism that is your read on the situation.

Somewhere I have read: "Don't bet if getting raised will make you throw up (Harrington?)."

Also: don't bet if you can only be called by a better hand, as might be the case given the limited information (in terms of your opponent).

Third: Ciaffone, in "Middle Limit Hold'em", recommends that you understand that with AK, you do have something to show down at the end. I have seen the slick win more than once unimproved, something that is quite likely heads up. In this case, the equity of a third bet is not that great: unlikely to be called unless up against a better hand. You may choose to pay the opponent off if he bets, given a fairly good hand heads up and the equity arising from understanding the kind of hand that apparently does turn your opponent into a calling station, at least for this hand.

The operative premise is based on your read of your opponent, and paying attention to the types of hands he shows down, plays aggressively, or folds is what will provide you equity in these kinds of situations.