Your erection should get harder, not softer, from any good PE routine, including hanging.
However, it is possible that your erection angle may diminish if you lengthen your ligaments substantially. I have not noticed a change in my erection angle, but I would actually welcome a decrease to 90-degrees (i.e., parallel to the floor when standing), since this makes the penis look longest.
By the way, many of these guys in the movies are probably having erection troubles from (a) being videotaped; (b) overdoing the pumping; and (c) having to keep their erections for hours upon hours during the typical porn shoot. It’s also possible some of them have had their ligs completly cut by a doctor (penile lengthening surgery), which generally does decrease erection angle.