Personal idea, not that useful in ibt than in GMAT, cause the questions are totally random selected in ibt while there are different levels of quetions in terms of difficulty in GMAT. The jj you are looking at when perparing your GMAT are mosty in the highest level, so it is a relatively narrow range, and that's why jj are especially important in GMAT. But that is not the case in ibt. The questions are still in a very very wide range in ibt test. So I don't think jj are useful for preparing ibt test.