Of course if you mortise out in the middle of the bench your back will bear the brunt in time. Front edge of the bench, just south of the vise, has always worked for me even on less than massive benches. One obviously wouldn't want to mortise on top of their shoulder vise so moving the party to the other side of the bench might be a good idea assuming the bench isn't against the wall and the bench doesn't have a tool well. A shortish bench that has a shoulder vise and tool well can be a little problematic with regard to real estate for mortising while standing upright.