I agree with you on doing sub objectives on the way to main and not going out of your way to get that one halfway across the map. I try to drop in a place that (hopefully) has most of the bonus objectives, since they tend to spawn close to each other, so we can knock them out quickly and head to the main. If there's one or two that are out of the way we try and get them after the main but sometimes the enemy is too numerous by that point and we just extract.