Depends on the popularity and availability of the car you are after, really. My Neon, was only going to be around 5-10 thousand made, and they sent the first couple thousand to FL and CA, to hit the always sunny crowd. SO on that car, I was working from MSRP. I could have gone the invoice route, but for that, I would had to wait til they filled the retail channel and all preorders were sold. Maybe a year after release. If its the brand new version of a popular model, you might have to go for MSRP. But for say a Mustang thats not a special edition, work from invoice.
Of course they will say "Have a nice day", and you wish them the same and go home. Just like a kid or a pet, you have to show them you are serious. And be willing to wait them out. Dont be mean, or get mad, or disparaging. That salesguy is under silly pressure. Just be nice and firm.
They will work from invoice, if they think that the only way to make a sale. See, invoice means they only make %3 profit. Here, take a read....
Dealer Holdback - Edmunds.com
And use the TMV tool for what you are after, see what they average person is paying. You will see it usually falls between invoice and MSRP. Now if you are after a limited edition (my Charger, first year of its type, only 5,000 or so made like it), you wont get far. I got lucky and the usual disclaimer about employee pricing not applying to SRT models. So my car that stickered at 43K was 38K out the door to me.