First, that concept, while true in theory, is kind of negated by the patrol car videos we have of officers getting beat down, shoot, etc. by folks who have consented to a search.
Calm suspects are good, what's even better is a back up officer doing nothing but watching the suspect(s).
Ever ask yourself why a guy with a trunk full of dope consented to your search? Common sense would tell us that if a person knew they could refuse consent, they would. Often it's because they are panic stricken, too stressed out to think of another alternative. Sometimes they are going with the flow, waiting for an opportunity to pounce. Either way, these folks are dangerous if you let your guard down for a moment.
https://www.youtube.com/watch?v=2FraE77l4fI