FWIW-
Of course the LW attacked from the rear and from the front. It is also fair to say they were scared sh**less (and rightfully so) every time they did it, and almost always lost some pilots in the process. One .50 in the head will ruin your whole day.
The point of the massed attack in the HO was to break up the formation, whereupon it would be easier to attack the single bombers. Tail chases were done as well, but a lot of that film you see shows a dead gunner protecting the rear of the B-17 (and the LW pilot knew the gunner was dead).