Shoot according to what the situation affords. This isn't exclusive to elk but most all hunting. Most people go to the range and shoot from the bench until their rifle is sighted then call it good. Practice in every position imaginable. Standing with pack on or off, prone unsupported, prone supported (on a pack, rock or something else), single kneeling, double kneeling, seated using knee for support (no bone on bone contact), using a tree to brace, all the position above but at steep angles, contorting your body into very uncomfortable positions, etc. I haven't shot from a bench in a long time and I shoot often. Also make sure your pack weight is mostly on your hip belt on the iliac crest so your arm and shoulder movement isn't as restricted. For bow hunting it's easy enough to shoot with a pack on if your pack is worn correctly and you practice. I will always prefer removing my pack if I can, but if not I'm still comfortable with taking a shot.