If I'm sitting around playing at home and close to the carpet, I lift by the stops. If I'm standing on stage, I lift by the shoulder strap. I've been lifting by the stops for years -- never had an issue.
When I started playing, I never used the stops to lift or carry the box. Then I saw several people do it, and thought it was okay. However, I won't do it again. All it takes is a little sideway pressure to damage the stop mechanism.