Sub-frame connectors

A while back I had mentioned installing subframe connectors on my Duster. I asked if bolt-on was OK for street use or if they needed to be welded as well. Several replies surprised me, as more than one poster mentioned subframe connectors needed to be installed with the car in street trim (engine, trans, etc installed as it will be run on the street) and with weight on the suspension; i.e NOT on jack stands.
This is surprising to me. Right now my A-body is up on jack stands with no suspension or drive drain, including torsion bars and K-frame. Obviously the perfect opp to crawl under and do the work, but totally against what these other guys are saying. So, if true, is the body 'pre-flexed' in the street trim with motor, etc and weight on the wheels? I'm not sure why this would make a difference. Right now the chassis is in a full-relaxed position-seems this would be the ideal time to install the SF connectors. Anybody else wanna weigh in on this? And what negative results would be derived from installing them now?