Melee should use on single targets, ranged on groups. But if the melee do not use it then its ok for ranged to step in. However, if ranged it's usually will be more beneficial to lb a large pack than a single boss and the ranged lb 1 and 2 is sometimes less powerful on a single target than just sticking to your rotation, so if there is no melee present, using it on pack pulls does more good in most cases.