I think it would be for the best for the Jets to pick their spots when they are going to use Cromartie as a return man. His most important job is going to be lining up against the other team's second best receiver on defensive snaps. Avoiding an injury has to be a consideration. Jason Sehron is the example people always mention in situations like this.
If the team needs a shot in the arm, a big return late like against the Colts, or is just in a big game where it needs to pull out all the stops to win, Cro is clearly a great option who is electric with it in his hands. In normal spots, I think the best bet is to let somebody athletic like Joe McKnight try and take advantage of Mike Westhoff's blocking schemes to replace Brad Smith.