This is all true ... BUT... When you give them the ground rules up front ( Which generally I like and agree with...) They then know there is going to be no more added profit in the finance office and they better make "Some" money on the actual deal. Most dealers make more money on the gingerbread fees and add on's and financing than they ever do on the actual selling price.... So you won't get the actual "Lowest" price on the vehicle, as the sales guy runs back and forth with the GM/finance manager.
The best position is to get into the finance office as soon as possible telling the sales guy you will make the deal if you can come to terms with Finance/GM. In that office is where the rubber meets the road. #1 rule in this office is don't be afraid to WALK. Have a # in mind that you researched and are willing to pay all in. They can add fee's etc sealant, finance it, loan shark loan it, and give you teddy bears, but this is your number and you are buying today if that # can be met. If its a FAIR # (Subjective and based on availability and consumer demand..) they will not let you walk out of that door.
I usually go in with the attitude that I don't HAVE to buy a car today, but if they will meet me somewhere in a fair zone in my mind, we will make a deal today. Remember a good deal is really only what you perceive it to be. The guy behind you could have paid a lot more or a lot less and you will never really know....
I wouldn't disagree if I were coming into a Chevy/Cadillac/GMC dealer cold and was playing "let's make a deal" with the salesman, his sales manager and then the GM. I had already been down that road with enough dealers to know what the best deal was. When you can get top dollar for your trade and $5,000+ off sticker on a '21 Lexus (not a grossly overpriced domestic) that just got on their lot and it's the LAST day of their fiscal year...I knew I got a fair deal. They offered the extended warranty and other crap, but I told them one of the main reasons I purchase Toyota vehicles is because their bulletproof. The finance guy just smilled and I wrote him a check for the money difference.